Guide · Boston Zoning Code, Article 32
Building below grade in Boston’s Groundwater Conservation Overlay District
Much of central Boston stands on filled land, and many of its older buildings stand on wood piles that last only while they stay under water. Article 32 of the zoning code, the Groundwater Conservation Overlay District (GCOD), is how the city keeps them there. This guide covers what triggers it, what the city asks for and when, and what it changes about a basement lowering or a new building’s excavation.
Plain-language guidance for planning, not legal or engineering advice. The engineer of record designs, and the zoning code, BWSC and the Boston Groundwater Trust have the last word.
Start here
Is the site in the overlay?
Type a Boston street address, paste a latitude and longitude, share your location or tap the map. The answer comes from the Planning Department’s published GCOD boundary, tested in your browser.[GCOD map layer]
The check runs in your browser against the published boundary. Nothing you type is sent anywhere.
- Capture and no-harm area
- No-harm area
A point is not a lot. If the lot runs close to the line, check the whole lot against the Article 32 map before relying on one point.[§ 32-2]
About the area labels. The city’s map layer still labels the areas it added in 2007 “No Harm Area”, and the check shows that label as a no-harm area.[GCOD map layer] The label dates from the 2007 text of Article 32, which asked only for the no-harm certification in the overlay mapped in the North End, the Bulfinch Triangle, the Central Artery district, the Fort Point waterfront and South Boston.[Art. 32, 2007 text] The current text has no such exception, and its map shows one district.[§ 32-2][§ 32-3] For a site in one of those areas, confirm with BWSC and the Boston Groundwater Trust which letters they will expect.
Permits, streets and ground-opening work inside the line: the full GCOD data module
In five points
The short version
The overlay is drawn over parts of Boston’s filled land, where many older buildings stand on wood piles. An address, not a neighborhood name, decides whether a site is in.[Amendment summary][§ 32-2][Trust: overview]
Four kinds of work trigger it: excavating to elevation 8 feet Boston City Base or lower for new or extended space, adding more than 50 square feet of ground-floor footprint, new or replacement paving, and substantial rehabilitation.[§ 32-3][§ 32-4]
It takes a certification from a Massachusetts registered engineer, in practice two letters: BWSC’s approval of a recharge system for 1 inch of rain over the impervious area, and the engineer’s no-harm letter covering groundwater on your lot and the lots next door.[§ 32-3][Trust: Art. 32 letters]
The letters go in with the Zoning Board of Appeal filing (dig-down, footprint and paving projects) or with the building permit application (substantial rehabilitation). The Trust says the board won’t schedule a hearing until it has both.[§ 32-3][§ 32-4][Trust: Art. 32 letters]
For the dig, the question is how to keep the hole dry without drawing down the water under the neighbors’ piles. The engineer of record answers it; cut-off walls, recharge and monitoring are the usual tools.[§ 32-3][Trust: overview]
01 · Why it exists
What the overlay is, and why it exists
Much of central Boston is filled land: sand and gravel laid over former mud flats. That fill could not carry heavy masonry buildings on its own, so builders drove wood piles down through it and the soft layer beneath, then set granite cap stones on top. The Boston Groundwater Trust says most buildings put up on filled land before the 20th century rest on wood piles.[Trust: overview][Groundwater Trust]
Wood that stays below the water table can last for centuries. When groundwater drops below the pile tops and air reaches them, microbes start to decay the wood. The connection between pile and cap stone fails, and the building above can settle unevenly, cracking walls and jamming doors and windows. The repair is underpinning, which the Trust describes as costly and disruptive work that can run for months.[Trust: overview]
The Boston Groundwater Trust, a nonprofit the City Council set up in 1986, watches the groundwater those foundations depend on and helps property owners comply with the overlay.[Groundwater Trust]
Article 32 exists to protect them. Its stated purpose is to prevent the deterioration of groundwater levels and promote their restoration, to protect areas and structures on wood pile foundations that are at risk of groundwater depletion, and to reduce runoff and water pollution.[§ 32-1]
Where it applies
The Trust lists the neighborhoods built on filled land as the Fenway, Back Bay, the South End, Bay Village, parts of Beacon Hill, Chinatown, the Leather District, the Bulfinch Triangle, the North End, the downtown waterfront, the Fort Point Channel section of South Boston and sections of East Boston.[Trust: overview] The overlay is a mapped line drawn over parts of that land, so check the address rather than going by the neighborhood name.[Amendment summary][§ 32-2]
Ground notes for two of those neighborhoods: Back Bay and Beacon Hill.
02 · Triggers
What triggers Article 32
Article 32 reaches a project when someone applies for a building permit for work inside the overlay that falls into one of four kinds. Since Text Amendment No. 487 took effect on October 28, 2025, the article sorts them into two routes: three go to the Zoning Board of Appeal (ZBA), and substantial rehabilitation goes through the building permit.[Art. 32][Zoning Commission][§ 32-3][§ 32-4]
ZBA · conditional use permit
Dig down
Building or extending space for people to use or pass through, mechanical equipment, laundry or storage (garages included) where the excavation reaches elevation 8 feet Boston City Base or lower. Digging only to build the Article 32 recharge system doesn’t count.
“…involves the excavation below grade to a depth equal to or below eight (8) feet Boston City Base…”
[§ 32-3.1(a)] ZBA · conditional use permit
Bigger ground-floor footprint
Building or extending a structure so that the lot area covered by the ground floor of each building grows by more than 50 square feet.
“…increase the Lot Area covered by the Ground Floor of each building on the Lot by more than fifty (50) square feet…”
[§ 32-3.1(b)][Art. 2] The code defines the Ground Floor as the lowest story with at least 65% of its height above grade.
ZBA · conditional use permit
Paving
New paving, or replacement paving of a surface that is already paved.
“Any new or replacement paving of an existing paved surface.”
[§ 32-3.1(c)] Building permit · no ZBA hearing
Substantial rehabilitation
Alterations or repairs within any 12 months that cost more than 50% of the building’s physical value, measured by its assessed value as of the January 1 before the filing.
“…Substantially Rehabilitate any structure.”
[§ 32-4.1(a)][Art. 2][Amendment summary] A project that needs zoning relief for anything else still goes to the Zoning Board of Appeal.
Elevation 8 is an elevation, not a depth
The dig-down trigger is measured on Boston City Base (BCB), the datum BWSC requires for site-plan elevations, not in feet below the sidewalk. How deep you can dig before you reach it depends on the ground level at your site.[§ 32-3][BWSC datum rule] Surveys drawn to another datum need converting; BWSC’s checklist puts BCB at the NGVD elevation plus 5.65 feet.[BWSC checklist]
The text looks at the excavation, so compare the lowest point you will dig to, footings, pits and sumps included, with elevation 8, not just the finished floor.[§ 32-3]
El. 8 ft BCB
Article 32’s dig-down trigger: excavation to this elevation or lower.[§ 32-3]
El. 7.5–8 ft BCB
The range the Charles River is typically held in, according to the Trust.[Trust: well & foundation data]
El. 5 ft BCB
Where much of the Trust’s recorded pile cut-off information sits. Some buildings are higher, some lower.[Trust: well & foundation data]
For orientation only. Groundwater and pile cut-offs vary from lot to lot; the Trust’s well readings and your engineer’s survey are what count for a site.
03 · Requirements
What the city requires
Every project caught by Article 32 needs a GCOD certification from a Massachusetts registered engineer that it meets two standards. The Boston Groundwater Trust describes the paperwork as two letters.[§ 32-3][§ 32-4][Trust: Art. 32 letters]
Letter one · Recharge
BWSC’s approval of a one-inch recharge system
- The project must capture at least 1.0 inch of rain over the part of the lot covered by structures or other impervious surface, in a system that lets it soak into the ground.[§ 32-3]
- BWSC reviews the design. The calculations must show that the infiltration structure plus its surrounding stone (void space counted at no more than 30%) holds more than one inch over the impervious area, stamped by a Massachusetts PE.[BWSC]
- In the overlay, green infrastructure in BWSC site plan review can’t use an underdrain or be designed for capture and reuse: the water has to go back into the ground.[BWSC storm drain rules]
- MassDEP tracks new infiltration systems as underground injection wells, and BWSC won’t approve a site plan before you have the UIC number (single-family homes are exempt). A system under a sidewalk or street also needs a Public Improvement Commission license.[BWSC storm drain rules][BWSC]
- Once it’s approved, BWSC issues an “Article 32 Compliance Only: Groundwater Storage & Recharge” letter, copied to the applicant, ISD, the Trust and the Planning Department.[Trust: Art. 32 letters]
- For a one-, two- or three-unit residence, the article accepts a system built to specifications approved by the Commissioner of Inspectional Services as presumptive compliance.[§ 32-3]
BWSC’s citywide site plan rules can ask for more: 1.25 inches over the impervious area for projects with 100,000 square feet of floor area or more.[BWSC site plans]
Letter two · No harm
A Massachusetts engineer’s no-harm certification
- A Massachusetts registered engineer certifies that the project results in no negative impact on groundwater levels within the lot or on adjacent lots, subject to the terms of any dewatering permit or of a cooperation agreement with the Boston Redevelopment Authority that sets groundwater standards during construction.[§ 32-3]
- The certification has to state the facts and opinions it rests on, and the applicant carries the burden of showing no negative impact. Meeting the recharge standard does not, by itself, count as meeting this one.[§ 32-3]
- The Trust says it goes in as a letter stamped by a professional engineer registered in Massachusetts, copied to ISD and the Trust, and to the Planning Department as well for projects in Article 80 review.[Trust: Art. 32 letters]
What this means for scheduling. Because the standard is tied to the terms of any dewatering permit, expect the certifying engineer to want the excavation-support and dewatering approach settled before signing.[§ 32-3] That pulls decisions a builder might leave for later into design: wall type, cut-off depth, pumping, recharge and monitoring.
Who does what
- Inspectional Services (ISD)
- Takes the building permit application; its plan review is where an Article 32 zoning refusal comes from. Its staff review the GCOD certification with BWSC and the Trust.[§ 32-3][Planning Dept. slides]
- Boston Water and Sewer Commission (BWSC)
- Reviews and approves the recharge system and issues the Article 32 compliance letter. Takes part in the staff review of the certification.[BWSC][Trust: Art. 32 letters][§ 32-3]
- Boston Groundwater Trust
- Tracks groundwater in the filled-land neighborhoods, takes part in the staff review, and receives copies of both letters.[Groundwater Trust][§ 32-3][Trust: Art. 32 letters]
- Zoning Board of Appeal
- Grants the conditional use permit that dig-down, footprint and paving projects need, conditioned on the staff review.[§ 32-3][boston.gov: ZBA]
- A Massachusetts registered engineer
- Signs the GCOD certification. BWSC also needs the recharge calculations stamped by a Massachusetts PE.[§ 32-3][BWSC]
- Your engineer of record
- Designs the excavation support, dewatering and any underpinning that the no-harm showing depends on.
04 · Timing
Where it sits in the permit process
Article 32 runs inside the ordinary building permit. For dig-down, footprint and paving projects, the Planning Department sets out the sequence like this:[Planning Dept. slides]
Apply to ISD for the building permit.[Planning Dept. slides]
Where Article 32 applies, ISD’s plan review refuses the permit and you appeal to the Zoning Board of Appeal for a conditional use permit.[Planning Dept. slides][§ 32-3]
File the GCOD certification with the appeal: BWSC’s recharge approval and the engineer’s no-harm letter, with the drawings and specifications for the recharge system.[§ 32-3][Trust: Art. 32 letters]
ISD, BWSC and the Trust review the certification. The Trust says the board won’t schedule the hearing until it has both letters, which go to ISD and the Trust by email before the hearing.[§ 32-3][Trust: Art. 32 letters]
Abutter notice and the hearing. Once the conditional use permit is granted, the building permit can issue.[Planning Dept. slides]
Substantial rehabilitation, since Oct. 28, 2025
The certification goes in with the building permit application, and ISD issues the permit only if the staff review finds it compliant. There is no Zoning Board hearing for Article 32 alone; a project that needs zoning relief for something else still goes to the board.[§ 32-4][Amendment summary][Planning Dept.][boston.gov: news]
Work backward from the filing
BWSC won’t approve the recharge site plan without a MassDEP UIC number, and its site plan approvals are valid for one year.[BWSC storm drain rules][BWSC site plans] The code has the certification filed with the appeal, and the Trust says the board won’t schedule a hearing without both letters, so start the recharge design and the no-harm analysis early.[§ 32-3][Trust: Art. 32 letters]
Everything else a below-grade job needs from the city is in our guide to Boston below-grade permits.
05 · The dig
What it means for below-grade construction
Article 32 doesn’t prescribe a method or set a drawdown number. It sets an outcome: no negative impact on groundwater levels on your lot or the lots next door.[§ 32-3] How a project gets there is the engineer of record’s design. What follows is typical of how designs approach it, not a design for any site.
Open excavation, pumped
The drawdown spreads past the lot line. The pile tops nearest the dig end up above the water.
Cut-off wall, pumped inside
The wall, taken into a less permeable layer, limits inflow, so the water outside stays close to where it was.
- Groundwater level
- Pile cut-off: the tops of the wood piles, under the granite cap stones
- Wood piles below the water
- Pile tops above the water, exposed to air
- Wall that holds back soil but not water
- Cut-off wall (sheet piles, secant piles or a slurry wall)
- Less permeable layer
Dewatering and recharge
Pumping an excavation lowers the water table around it, and the drawdown doesn’t stop at the lot line. The Trust warns that even a small sump pump can lower groundwater under neighboring properties.[Trust: overview]
So designs in the overlay limit how much is pumped and for how long: a cut-off wall to reduce inflow, pumping inside it, and sometimes putting pumped water back into the ground nearby so levels outside hold. Sending pumped water to BWSC’s sewers or drains can also need a BWSC dewatering discharge permit.[BWSC storm drain rules]
Permanent pumping is the harder case. A basement that stays dry only while a pump runs keeps drawing groundwater down for as long as it runs, so below the water table designers often make the new structure watertight instead.
Pile cut-off elevations: the number next door
A building’s pile cut-off elevation is the level at which its wood piles were cut to take the cap stones. The Trust notes it can vary substantially between neighboring buildings, and even within one building, and that the higher the cut-off, the more vulnerable the piles are to lower groundwater.[Trust: well & foundation data]
Gather what is known before design. The Trust’s building-foundation map records foundation type, cut-off elevation where available and whether a building has been underpinned; old permit files can hold more.[Trust: well & foundation data][ISD permit records] Where it matters and nobody knows, the Trust describes engineers digging a test pit beside the building to see the piles, their condition and cut-off, and compare them with groundwater readings.[Trust: overview]
Watertight cut-off walls versus open pumping
Soldier piles and lagging hold back soil, not water, so a dig below the water table behind them is kept dry by pumping, and the drawdown spreads. A near-watertight wall (interlocking steel sheet piles, overlapping secant piles or a slurry wall) taken down into a less permeable layer, where the ground has one, cuts off most of the inflow, so only the water inside the wall is pumped.
Which one fits depends on the soil profile, the depth, the neighbors and how much vibration they can take. The engineer weighs how the wall goes in as well as what it is.
Shoring & support of excavationUnderpinning next to wood-pile buildings
When a dig goes below a neighbor’s foundation, that foundation needs support either way: a stiff support-of-excavation wall along that side, underpinning that carries it down below the dig, or both.
Next to wood piles, underpinning also has to respect the water. The Trust describes repairing decayed piles by cutting the rotten length away to below the groundwater level expected in future, replacing it with steel and encasing the steel in concrete.[Trust: overview] Drilled micropiles are another way to carry a foundation past the soft layers. Underpinning a neighbor’s building needs the neighbor’s agreement.
Monitoring before, during and after
Observation wells are how the Trust watches groundwater, and how a project can show its levels held. The Trust publishes readings for its active wells, and history for decommissioned ones, on an online map with downloads.[Trust: well & foundation data]
A typical program takes baseline readings before work starts, reads the wells often while the hole is open against trigger levels the engineer sets, and keeps reading afterward until levels settle. Put project wells where the excavation support and site logistics won’t destroy them.
New wells in a sidewalk or street need a Public Improvement Commission application, and the Trust decides whether they are handed over to it when the license ends. Trust wells already in the sidewalk must be preserved during the work; the Trust puts that on the contractor.[Trust: well permits]
06 · Checklist
A builder’s checklist
For a basement lowering, an addition or a new building inside the overlay. Adapt it with your engineer; the order matters more than the wording.
01Before design
- Run the address through the check on this page. If the lot runs close to the line, check the whole lot against the Article 32 map, not just one point.[§ 32-2]
- Get existing grade and the lowest point you will dig to (footings, pits and sumps included) stated in Boston City Base. If the dig for new or extended space reaches elevation 8 feet or lower, it is a dig-down project.[§ 32-3][BWSC datum rule]
- Check the other triggers: more than 50 square feet of new ground-floor footprint, new or replacement paving, or work costing more than half the building’s assessed value within 12 months.[§ 32-3][Art. 2]
- Pull the nearest observation-well readings and what is known about the neighbors’ foundations: type, pile cut-off elevation, past underpinning.[Trust: well & foundation data][ISD permit records]
- Commission borings and a geotechnical report that cover groundwater levels and any less permeable layer a cut-off wall could reach.
02Before you file
- Settle the excavation-support and dewatering approach with the engineer of record, with the no-harm certification in mind.[§ 32-3] Shoring & support of excavation
- Design the recharge system for 1 inch of rain over the impervious area and submit it to BWSC. Apply early for the MassDEP UIC number, since BWSC won’t approve the site plan without it. A system under a sidewalk or street also needs a Public Improvement Commission license.[§ 32-3][BWSC][BWSC storm drain rules]
- Collect BWSC’s approval letter and the engineer’s no-harm letter, and copy both to ISD and the Boston Groundwater Trust.[Trust: Art. 32 letters]
- File them with the appeal to the Zoning Board of Appeal (dig down, footprint, paving) or with the building permit application (substantial rehabilitation).[§ 32-3][§ 32-4] The other below-grade permits
03Before you dig
- Agree protection and access with the neighbors, and record the condition of their buildings before work starts. Excavating next to a neighbor
- Take baseline groundwater readings. New wells in a sidewalk or street need a Public Improvement Commission application, and existing Trust wells there must be protected.[Trust: well permits][PIC]
- If pumped water will go to BWSC’s sewers or drains, check whether BWSC’s dewatering discharge permit applies; it covers certain discharges to both.[BWSC storm drain rules]
04While you dig
- Read the wells on the schedule the engineer sets, and act on the trigger levels in the design.
- Keep pumping inside the cut-off wherever the design allows, and run any recharge as designed.
05After
- Keep reading the wells until levels settle back, and hand the records to the engineer.
- Send BWSC the as-built plans, submitted by a Massachusetts professional engineer, within 30 days of completion.[BWSC site plans]
07 · Questions
Questions builders ask about the GCOD
What is Boston’s Groundwater Conservation Overlay District (GCOD)?
It is a zoning overlay created by Article 32 of the Boston Zoning Code. It covers parts of Boston’s filled land, where many older buildings stand on wood piles that rot if groundwater drops. Projects it catches must recharge rainwater into the ground and show they will not lower groundwater on their own lot or the lots next door.
Does a basement lowering trigger Article 32?
It does if the site is in the overlay and the new or extended space (living space, mechanical, laundry, storage or a garage) needs excavation to elevation 8 feet Boston City Base or lower. That is an elevation on the city’s datum, not a depth below the sidewalk, so have your surveyor compare the bottom of the dig with it.
Sources:[§ 32-3][BWSC datum rule]
What does Article 32 require?
A certification from a Massachusetts registered engineer that covers two standards. In practice that means two letters: BWSC’s approval of a recharge system that captures 1 inch of rain over the lot’s impervious area, and the engineer’s stamped letter certifying no negative impact on groundwater levels on the lot or adjacent lots.
Do I need a Zoning Board of Appeal hearing?
For dig-down, ground-floor footprint and paving projects, yes: Article 32 requires a conditional use permit, and the Boston Groundwater Trust says the board won’t schedule the hearing until it has both letters. Since October 28, 2025, substantial rehabilitation projects file the certification with the building permit instead, unless another zoning issue sends them to the board.
Sources:[§ 32-3][§ 32-4][Trust: Art. 32 letters][Amendment summary]
Can I pump groundwater during construction in the GCOD?
Article 32 does not ban dewatering. It requires no negative impact on groundwater levels on your lot or adjacent lots, subject to the terms of any dewatering permit. Designs typically answer that by limiting inflow with a cut-off wall, keeping pumping short, recharging where needed and reading nearby wells, as the engineer of record specifies.
Sources:[§ 32-3][Trust: overview]
What is a pile cut-off elevation?
It is the level at which a building’s wood piles were cut off to take their granite cap stones. The piles stay sound while groundwater stays above it. The Boston Groundwater Trust notes that cut-off elevations can vary substantially between neighboring buildings, so what matters next to an excavation is the neighbors’ actual cut-offs, not a neighborhood average.
Where can I find groundwater levels near my site?
The Boston Groundwater Trust publishes readings from its observation wells on an online map, with downloadable data for active wells and history for decommissioned ones. The Trust tracks more than 800 wells, through scheduled manual readings and electronic sensors.
Does Earth Support Corp. provide the Article 32 certification?
No. The certification comes from a Massachusetts registered engineer, and the excavation support, dewatering and underpinning are designed by the project’s engineer of record. Earth Support Corp. is a specialty subcontractor: we price and install excavation support, underpinning, micropiles and tiebacks to that design, for builders, GCs, developers and engineers.
Sources:[§ 32-3]
08 · Sources
Sources
Checked against the current Article 32 (Text Amendment No. 487, effective October 28, 2025) and the agency pages below. Every regulatory statement on this page links to one of them. Where agency practice isn’t written down, the page says to confirm with BWSC or the Boston Groundwater Trust.
- Boston Zoning Code, Article 32, Groundwater Conservation Overlay District (Text Amendment No. 487, effective October 28, 2025)
City of Boston Zoning Code, published by Municode
- Article 32, Section 32-1, Statement of Purpose
City of Boston Zoning Code, published by Municode
- Article 32, Section 32-2, Designation of Groundwater Conservation Overlay Districts (the Article 32 Map)
City of Boston Zoning Code, published by Municode
- Article 32, Section 32-3, Dig Down, Lot Coverage, and Paving Projects
City of Boston Zoning Code, published by Municode
- Article 32, Section 32-4, Substantial Rehabilitation
City of Boston Zoning Code, published by Municode
- Article 2, Section 2-1, definitions of “Ground Floor” and “Substantially Rehabilitate”
City of Boston Zoning Code, published by Municode
- Article 32 as amended April 25, 2007 (superseded)
City of Boston
- Groundwater Conservation Overlay District layer (Zoning Viewer overlays)
Boston Planning Department GIS
- Updating the Groundwater Conservation Overlay District
Boston Planning Department
- GCOD Zoning Text Amendment Summary (August 2025)
Boston Planning Department
- Groundwater Conservation Overlay District (GCOD) Updates, public meeting presentation, August 19, 2025
Boston Planning Department
- Zoning Commission minutes, October 22, 2025 (Text Amendment Application Nos. 537–538 approved)
Boston Zoning Commission
- Groundwater Overlay District (site plan requirements)
Boston Water and Sewer Commission
- Storm Drain Requirements (GCOD, UIC registration, dewatering discharge permits)
Boston Water and Sewer Commission
- Site Plan Requirements (infiltration volumes, approvals, as-built plans)
Boston Water and Sewer Commission
- General Requirements: elevations relative to Boston City Base
Boston Water and Sewer Commission
- Site Plan Review Checklist (item A6: elevations in Boston City Base, NGVD + 5.65)
Boston Water and Sewer Commission
- Article 32/GCOD: the recharge approval letter and the no-harm letter
Boston Groundwater Trust
- Overview: filled land, wood piles and what happens when groundwater drops
Boston Groundwater Trust
- Groundwater & Building Foundation Data
Boston Groundwater Trust
- Groundwater Observation Well Permitting
Boston Groundwater Trust
- Boston Groundwater Trust: mission and monitoring network
Boston Groundwater Trust
- Zoning Board of Appeal
City of Boston Inspectional Services
- How to find historical permit records
City of Boston Inspectional Services
- Public Improvement Commission
City of Boston Public Works