Earth Support Corp

Ground conditionsBoston

Is your site in Boston's groundwater district?

Boston’s Groundwater Conservation Overlay District covers 2,273 acres in 10 mapped areas. Inside it, Article 32 of the zoning code asks two things of a project: keep the first inch of rain on the site, and show the work won’t lower groundwater next door. Check a site against the published map, read what the rule asks in plain words, and see how much building goes on inside the line.
  • Snapshot · as of Sep 16, 2026
  • 663,166 records
  • Boston: the overlay’s 10 mapped areas; building permits 2006–2026
  • 2 sources · method

Permits inside since Jan 1, 2024

22,502

20,790 in capture and no-harm areas, 1,712 in no-harm-only areas; issued through Sep 16, 2026

Share of the city's mapped permits

25.1%

162,684 of 647,435 permits with a map point, 2006–2026

District area

2,273 acres

10 mapped areas, adopted 2006, 2007 and 2021

Permits with a map point

97.6%

15,731 cannot be placed, so every inside count is a floor

Data as of Sep 16, 2026.

Read this first. 647,435 of 663,166 permits (97.6%) carry a usable map point; the other 15,731 cannot be placed inside or outside the overlay, so every count below is a floor. Permit data as of Sep 16, 2026. This page explains the rule and measures the public record. It is not legal or engineering advice about a particular site.

01 · The rule

What the overlay is, and what it asks of a project

Much of this part of Boston stands on filled land, and many of its older buildings rest on timber piles: tree trunks driven into the fill. Wood lasts for centuries while it stays under water and rots once air reaches it. When groundwater drops, pile tops dry out and decay, and the building above can settle. Article 32 of the Boston Zoning Code draws the Groundwater Conservation Overlay District around areas where that matters, and adds rules for work inside it.

The published map carries two kinds of area. Which one your site is in decides which duties apply.

  • Capture and no-harm areamapped as “1" Capture and No Harm Area”

Both Article 32 duties apply: capture and infiltrate the first inch of rain on the site’s impervious area, and file a registered professional engineer’s showing that the work will not lower groundwater on neighbouring lots.

Areas
5
Acres
2,108
Adopted
2006, 2021
  • No-harm-only areamapped as “No Harm Area”

The no-harm showing applies: a registered professional engineer must show the work will not lower groundwater on neighbouring lots. The one-inch capture duty is not mapped here.

Areas
5
Acres
164
Adopted
2007

Duty one

Capture the first inch

A system that holds the first inch of rain falling on the lot’s roofs and paving, and lets it soak into the ground rather than run to the storm drain. On a tight urban lot that often means drywells or infiltration chambers under the site.

Duty two

Show no harm

A registered Massachusetts professional engineer (a licensed PE) shows that the project will not lower groundwater on neighbouring lots. Excavation is usually where that showing is hardest, because that is where water gets pumped.

Before you rely on it

Read Article 32 itself

Which projects trigger the article, the exemptions and the review route are set out in the zoning code, and they are what count. This page is a plain-language summary to plan around, not a substitute for the code, the city or your own engineer.

▸ The words you will hear, defined
Overlay district
A zoning layer drawn over the ordinary zoning map. It adds rules for the land inside it; it does not replace the base zoning.
Groundwater
The water filling the gaps in the soil below the water table.
Timber (wood) piles
Tree trunks driven into soft ground to carry an older building. Wood kept under water lasts for centuries; once the water drops and air reaches it, it rots.
Recharge, infiltration
Putting water back into the ground: rain led into drywells, leaching chambers or other systems that let it soak away on site.
Dewatering
Pumping groundwater out of or around an excavation so it can be dug and built in the dry.
Cut-off wall
A near-watertight wall around an excavation (interlocking steel sheets, overlapping concrete piles or a slurry wall) that limits how much groundwater flows in.
Underpinning
Carrying an existing foundation down to deeper or firmer support, usually in short sections, so digging next to it does not undermine it.
Observation (monitoring) well
A small pipe set in the ground so the water level can be read, before, during and after construction.

02 · Check a site

Is my site in it, and which area?

Type a Boston street address, paste a latitude and longitude, share your location or tap the map. The answer comes from the published boundary, tested in your browser.

or tap the maptry

The check runs in your browser against the published boundary. Nothing you type is sent anywhere.

Capture and no-harm area, adopted 2006. 1,652 acres, mostly ZIP 02116.Capture and no-harm area, adopted 2021. 221 acres, mostly ZIP 02128.Capture and no-harm area, adopted 2021. 113 acres, mostly ZIP 02118.Capture and no-harm area, adopted 2021. 73 acres, mostly ZIP 02215.No-harm area, adopted 2007. 65 acres, mostly ZIP 02210.No-harm area, adopted 2007. 54 acres, mostly ZIP 02109.Capture and no-harm area, adopted 2021. 49 acres, mostly ZIP 02109.No-harm area, adopted 2007. 38 acres, mostly ZIP 02114.No-harm area, adopted 2007. 6 acres, mostly ZIP 02110.No-harm area, adopted 2007. 1 acres, mostly ZIP 02109.
  • Capture and no-harm area
  • No-harm-only area

The outline is the city’s published boundary, all 2,032 vertices, not simplified; coordinates are rounded to 5 decimal places (about 0.82 m). Addresses are matched against 30,376 street addresses that have carried a city permit inside the district or within about 1.3 km of it. That file holds a street, a house number and a map point, nothing else, and loads only when you use the box. The faint dots on the map are a sample of 1,688 of those addresses, there to show the street pattern. Source: Boston Planning Department (BPDA) GIS.

▸ All 10 mapped areas
Area typeAdoptedAcresMost permits carry ZIPPermits, 2006–2026
Capture and no-harm area20061,65202116119,666
Capture and no-harm area2021221021289,931
Capture and no-harm area2021113021182,844
Capture and no-harm area202173022152,752
No-harm-only area200765022106,072
No-harm-only area200754021093,548
Capture and no-harm area2021490210914,173
No-harm-only area200738021143,441
No-harm-only area2007602110214
No-harm-only area200710210943

03 · Building inside the line

25% of Boston’s mapped permits fall inside the overlay

Of the 647,435 permits in the city’s dataset that carry a map point (2006–2026), 162,684 sit inside the district: 149,366 in capture and no-harm areas and 13,318 in no-harm-only areas. Only 1.3% of them are for work that opens the ground; section 4 looks at those.

Permits inside the overlay each year, 2010–2025

An average of 8,565 a year in 2021–2025, against 9,788 in 2010–2014 (−12%). The boundary was adopted in stages (2006, 2007 and 2021); every permit, whatever its year, is placed against today’s boundary. These counts describe geography, not which rules applied at the time.
02,0004,0006,0008,00010,00012,00020102012201420162018202020222024
n = 156,485 permitsSource: City of Boston open data
▸ Show the numbers
YearInsideCitywide, mappedShare
20107,75230,05125.8%
20119,09234,31626.5%
201210,06036,24727.8%
201310,79339,09927.6%
201411,24442,38126.5%
201511,02042,31026.1%
201611,70244,30126.4%
201711,67844,90926.0%
201811,21443,71125.7%
201911,51544,58925.8%
20207,58831,71623.9%
20218,68037,46123.2%
20229,01839,30122.9%
20238,62937,98922.7%
20248,32536,75522.7%
20258,17536,26922.5%

The overlay’s share of the city’s mapped permits, 2010–2025

26.8% of the city’s mapped permits in 2010–2014, 22.8% in 2021–2025.
0%5%10%15%20%25%30%20102012201420162018202020222024
n = 621,405 permits citywideSource: City of Boston open data

Where inside: by ZIP code

Permits inside the overlay by the ZIP on the permit, 2006–2026. ZIPs with fewer than 100 are left off.
  1. 02116
    46,34128%
  2. 02118
    26,50016%
  3. 02115
    19,94512%
  4. 02215
    14,7179%
  5. 02128
    9,9316%
  6. 02109
    9,5806%
  7. 02110
    7,8915%
  8. 02111
    6,2534%
  9. 02210
    6,0724%
  10. 02199
    4,9013%
  11. 02114
    4,1363%
  12. 02108
    3,5202%
  13. 02113
    1,7311%
  14. 02120
    1,0381%
  15. 02119
    1270%
n = 162,683 permitsSource: City of Boston open data

The busiest streets inside the line

Permits inside the overlay by street, 2006–2026, top 20.
  1. Boylston St
    12,319
  2. Commonwealth Ave
    7,913
  3. Beacon St
    6,890
  4. Huntington Ave
    6,821
  5. Newbury St
    5,694
  6. State St
    3,918
  7. Marlborough St
    3,602
  8. Tremont St
    3,441
  9. Harrison Ave
    3,383
  10. Washington St
    3,193
  11. Clarendon St
    2,877
  12. Massachusetts Ave
    2,837
  13. Brookline Ave
    2,552
  14. Columbus Ave
    2,305
  15. Oliver St
    2,137
  16. Franklin St
    2,080
  17. Berkeley St
    1,937
  18. Shawmut Ave
    1,677
  19. High St
    1,631
  20. Congress St
    1,385
Source: City of Boston open data

04 · Opening the ground

Among permits that open the ground, 33% inside the overlay are filed as excavation, against 10% outside

Most permits never open the ground. Only 1.3% of permits inside the overlay are for ground-opening work, against 2.7% outside it, so a raw comparison of rates would mislead. The fair comparison is among ground-opening permits only: 2,100 inside the overlay and 12,978 outside it.

Among ground-opening permits only: inside the overlay against outside it

Ground-opening means the city filed the permit under one of these work categories: erect, addition, excavation borings test pits, demolition - exterior, new construction, removal of structure, garage and trench. Groundwater language is matched in the applicant’s own description of the job, at the start of a word. All years in the load, 2006–2026.

Filed as “Excavation Borings Test Pits”the city’s own work category

Inside
32.9% · 690 of 2,100
Outside
9.9% · 1,281 of 12,978

Filed as “Excavation Borings Test Pits”: 32.9% of ground-opening permits inside the overlay (690 of 2,100), against 9.9% outside it (1,281 of 12,978); 3.3× as common.

3.3× as common inside, among ground-opening permits.

Narrative mentions groundwater workdewatering, recharge, wells, borings and similar

Inside
28.7% · 602 of 2,100
Outside
8.1% · 1,050 of 12,978

Narrative mentions groundwater work: 28.7% of ground-opening permits inside the overlay (602 of 2,100), against 8.1% outside it (1,050 of 12,978); 3.5× as common.

3.5× as common inside, among ground-opening permits.

n = 2,100 inside, 12,978 outsideSource: City of Boston open data
▸ Show the numbers
Ground-opening categoryInsideOutside
Erect5665,512
Addition3093,096
Excavation Borings Test Pits6901,281
Demolition - Exterior2161,209
New construction185927
Removal of structure115836
Garage18112
Trench15
What this does and does not say. It describes what was filed, not what was built or what a site needed. Permit descriptions are short and uneven, so language counts are a floor. And it is not a forecast for any one project: your site’s ground, depth and neighbours decide that. Groundwater terms matched: “recharge”, “infiltrat”, “monitoring well”, “observation well”, “groundwater”, “ground water”, “dewater”, “well point”, “wellpoint”, “test pit”, “test boring”, “boring”, “article 32”, “gcod”, “conservation overlay”, “water table”, “drywell”, “dry well”, “leaching”.

05 · Excavation and foundations

What the overlay changes about the hole in the ground

None of this rules out basements, garages or deep foundations. It changes how a hole can be kept dry without taking the neighbours’ groundwater with it. These are the questions that come up; the answers for a given site belong to its geotechnical engineer and the engineer making the no-harm showing.

Open cut and pumping, or a cut-off wall

The simplest way to dig below the water table is often to pump the water down around the hole. That drawdown does not stop at the lot line, and ruling out harm to neighbouring groundwater is exactly what the overlay asks the engineer to show. So designs inside it often lean toward support of excavation that holds water back: interlocking sheet piles or other near-watertight walls, taken down into a less permeable layer where the ground has one, so only the water inside the wall has to be pumped.

Sheet pile walls & cofferdams

Recharge, during and after

Where pumping can’t be avoided, the water taken out can be returned to the ground nearby (recharged) so levels outside the excavation hold. The permanent one-inch capture system is recharge too, and it often wants the same part of the site as the excavation and the foundations, so it is cheaper to lay the two out together than to redesign one around the other late.

Monitoring

Observation wells are the usual way to show that levels hold: a baseline before work starts, readings while the hole is open, and readings after. Readings like these are often part of the record a no-harm showing relies on, so put the wells where excavation support and site logistics will not destroy them.

Neighbours on wood piles

Digging next to a building on timber piles carries two risks: the excavation can undermine its foundation, and any drawdown can dry out the tops of its piles. The usual answers are a stiffer, near-watertight support wall along that side, underpinning the neighbour’s foundation, or both, with a condition survey of the neighbour before work starts.

Underpinning

Where these numbers come from

Method, sources & caveats

What is counted. Every permit in the city’s approved-building-permits dataset, 663,166 of them issued 2006-09-26 to 2026-09-16, placed inside or outside the published boundary by its map point. The city published the extract on Sep 16, 2026. A permit with no usable point is left out of every count and every share, never assumed outside.

Checked twice. The same classification was run separately over the same permits, and every count the two share, from the geocoded total to the ground-opening comparison, matched exactly before this page was built.

Categories and language. The city files each permit under a fixed work category; that label is used as it stands and is never read as a description. Groundwater terms are matched only in the applicant’s own text. The one inside-against-outside rate on this page is taken among ground-opening permits, because across all permits the overlay’s mix of work is different enough to mislead.

What is published, and what is not. No names, permit numbers, parcel IDs, valuations or permit descriptions. The address list behind the site check holds only a street, a house number and a map point for addresses that have carried a permit.

  • 647,435 of 663,166 permits (97.6%) carry a usable map point. The other 15,731 cannot be placed inside or outside the overlay, so every “inside” count here is a floor, not a census.
  • The boundary was adopted in stages (2006, 2007, 2021). Every permit, whatever its year, is placed against today’s boundary: the counts describe geography, not which rules applied when the permit was issued.
  • Ground-opening permits are those the city files under one of 8 work categories: Erect, Addition, Excavation Borings Test Pits, Demolition - Exterior, New construction, Removal of structure, Garage, Trench. Groundwater language is matched in the applicant’s narrative only, at the start of a word, never inside a category label.
  • The raw share of permits that open the ground at all is lower inside the overlay (1.3%) than outside it (2.7%): the two areas file different mixes of work. So no inside-versus-outside rate is quoted except among ground-opening permits (2,100 inside, 12,978 outside).
  • Declared valuation is not analysed here: it is self-reported, sometimes a placeholder such as $1, and too noisy to compare.
Data as of
Sep 16, 2026
Generated
Sep 17, 2026
Records
663,166
Refresh
Frozen snapshot · re-synced when the city’s extract is reloaded

Published by Earth Support Corp as a public resource. Regional statistics for planning and curiosity. They are not an engineering report, a bid recommendation or a statement about any particular site, firm or project. Spot an error? Tell us and we will correct it. Full method: methodology.

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