As a Boston homeowner, landscape architect, and housing advocate, I wanted to know what building an ADU would actually require — and, before that, whether one was even possible where I live. The information is all public, but it's spread across zoning articles, assessing records, and program pages that each hold one piece of the answer. Putting it together for a specific lot means doing that work yourself, or paying someone to do it before you know whether the idea is worth pursuing at all.
That felt backwards to me. "Could this work here?" should be the cheap question. So I built the tool I'd wanted: something that looks at a parcel and shows you what an ADU there might look like — how much room the lot has, what it might rent for, what value it might add, and what it would likely cost to build.
It's for Boston homeowners weighing the idea, and for buyers sizing up a property's potential before they commit. It won't tell you what you're allowed to build — no map honestly can, and the sections below are careful about where that line falls. It's a first look, not an answer.
I'm providing this tool for free, and I'll maintain it as best I can. If you find it helpful, please consider supporting the work and buying me a coffee.
— Mark Warfel Jr.
Everything below explains how the map works and where its numbers come from, so you can decide how much weight to give them.
What this map shows — and what it doesn't
The map screens Boston's 1-, 2-, and 3-family parcels for physical ADU potential: roughly how much room a lot has, based on public assessor and zoning data. It does not tell you whether an ADU is legally allowed on your property, and it isn't a substitute for a site visit, an architect, a surveyor, or a conversation with the City.
Each parcel's panel ends with its Parcel ID, linked to the City's zoning viewer. That's the handoff: the viewer carries the authoritative zoning record and links onward to the assessor's card for the property. When this map and the City disagree, the City is right.
Boston's three kinds of ADU
The City sorts ADUs into three types, and this map uses the same words. An internal ADU is carved out of space you already have — a basement, an attic, splitting an existing unit. An attached ADU adds on to the house: another floor on top, or a new wing off the side or rear. A detached ADU is a separate building in the yard, which can also mean adapting a garage you already have.
Only the detached case really depends on how much yard you have, which is why that's what the map screens for. Attached ADUs extend less into the yard than a freestanding building, and internal ones need no yard at all — so the options don't run out on a small lot, they just shift toward the house. Boston's own ADU guidebook is organized this way: it sorts homeowners by lot size first, then shows which approaches suit each.
To learn more about what each type involves, start with the City's ADU program page — or see the resources below.
What the colors mean
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Larger lot — detached, attached, or internal. The lot appears to have enough depth, width, and setback clearance for a detached backyard structure.
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Medium lot — detached would be tight; attached or internal. There may be room for a backyard structure, but it's close. Worth a site visit or a survey before assuming it'll work — and worth asking about a garage conversion or a side addition instead.
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Smaller lot — attached or internal. The lot doesn't appear to have room for a detached structure, but converting existing space (basement, attic, splitting a unit) or building onto the house may still be possible.
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ADU rules cover 1–3 family homes — the parcel didn't enter the screening, because the City's ADU rules start from the home on the lot: "Internal ADUs may be built on all owner-occupied lots that contain 1-, 2-, or 3-family homes." The panel names what the assessor records the property as — a condominium, an apartment building, vacant land, tax-exempt or commercial property — so you can see which part didn't match, and correct it if the record is wrong.
The type lists above are a starting point for a conversation with a an architect or builder, not a finding about your property — see how the sizes are grouped for what was actually screened.
Falling outside the screening isn't always the same thing. For a condominium it lines up with a real City rule (see what Boston allows); for a vacant lot it just means there's no home to add to yet; for a 4-family or a shop it's a building this program doesn't cover. What the grey never means is that we checked your property and it failed.
Assessor records also go stale, and ownership structures change — a condo can be de-converted, a lot can be built on. And owner-occupancy is required but not something this map checks, so a parcel that was screened isn't confirmed eligible either. Both directions are questions for the City, not answers from us.
The badges on an address
A parcel's detail panel may show one or more small badges under its address — corner lot, no street frontage, irregular shape. These note where a parcel's geometry is unusual enough that the screening's standard assumptions may not hold cleanly: a corner lot has two front setbacks rather than one, a lot without street frontage raises access questions, and an irregular outline means a rectangle-based estimate of the buildable yard is rougher than usual.
They are prompts to look closer, not disqualifiers, and they say nothing about whether an ADU is allowed. A flagged parcel is often perfectly workable — Boston's own ADU guidebook treats a corner lot as an opportunity, since the second street frontage can suit an attached ADU. Treat a badge as a question for your designer or surveyor.
What Boston allows by right
This map doesn't tell you what's permitted — but the rules are worth knowing before you read anything above as encouragement, because the three ADU types are treated very differently.
Internal ADUs are the permissive case. In the City's words: "Internal ADUs may be built on all owner-occupied lots that contain 1-, 2-, or 3-family homes." Adding floor area or altering the exterior can still trigger zoning review.
Attached and detached ADUs are not. The City says they are "only allowed to be built without special zoning approval in Mattapan" — everywhere else in Boston they need zoning relief, which in practice means a variance from the Zoning Board of Appeal.
Ownership structure matters as much as building type. The City states plainly: "Condominiums and LLCs are ineligible for ADUs." So the question isn't only what's on the lot but how it's held — an owner-occupied three-family under single ownership is the eligible case, while the same building converted to condominiums is not. That's why this map screens by the assessor's record of the property rather than by the shape of the building.
Massachusetts' statewide ADU law does not apply in Boston. The 2024 Affordable Homes Act legalized ADUs by right in single-family districts across the Commonwealth — but it amended M.G.L. c. 40A, the Zoning Enabling Act, and Boston draws its zoning authority from Chapter 556 of the Acts of 1956 instead. The City says so itself. If you've read that Massachusetts now allows ADUs by right, that change did not reach Boston; the City's own rules above are what govern.
Boston's rules are also actively changing — the Planning Department's Neighborhood Housing initiative is redrawing zoning across several neighborhoods. Check with the City before relying on any of this.
Land use vs. zoning
The panel shows two separate facts that are easy to conflate: your parcel's assessor land-use code (e.g. "1-family," from the City's assessing data) and its zoning subdistrict (e.g. "2F-5000"). They come from different systems and don't always agree — both are shown so you can check both, not so one can stand in for the other.
The difference is load-bearing here. Whether a parcel gets screened at all comes from the land-use record, since the City's ADU rules start from the home on the lot. How much room the screen thinks you have comes from zoning, via the setbacks for your subdistrict. So a single zoning subdistrict contains both screened and unscreened parcels — that isn't an inconsistency.
Where the numbers come from
Every estimate on this map traces to a published assumption — never an invented figure. Each of the three below is versioned separately, because they come from different sources and change on different schedules.
How property sizes are grouped
The map's three colors sort properties by how much room they have, following the City's ADU guidebook, which asks homeowners how much space is on their lot before suggesting any approach. Only the detached case was actually screened, because it's the one that depends on open yard: an attached ADU extends less into the yard than a freestanding building, and an internal one needs no yard at all. So the attached and internal options listed against each color follow from the detached result rather than from separate tests of their own — treat them as worth asking about, not as findings.
Parcel screening
Screening looks at lot size, home size, principal-building setbacks (with a fallback when subdistrict data is missing), and lot-shape flags. The backyard-ADU size you see in each panel's headline is capped at 900 sq ft or 50% of the home's size, whichever is smaller. This logic is versioned as 2026-07a.
Rent estimates
Where a detached-ADU size estimate exists, the panel shows a rent range from HUD's FY2026 Small Area Fair Market Rents (SAFMR) for Boston, matched to each neighborhood by intersecting ZIP code and neighborhood boundaries. Ranges reflect the low–high SAFMR span across the bedroom size(s) that footprint could plausibly be — never a single invented point figure. SAFMRs sit around the 40th percentile of an area's rents, so actual market rents often run higher. This table is versioned separately from the parcel screening, as HUD-FY2026-SAFMR.
Value added & build cost
The value figure uses income capitalization — the standard way a rentable unit is valued. Take the annual rent, subtract 40% for operating costs, and divide by a 6.25% capitalization rate. The rate is drawn from what Boston two- and three-family property actually trades at (roughly 5.5–7%), not from institutional apartment buildings, which trade much tighter and would flatter the result. The 40% is if anything generous to the estimate: built up from Boston's FY2026 residential tax rate, insurance, maintenance and vacancy, a typical unit lands nearer 31% — partly because the City requires owner-occupancy, so there's no management fee. The range you see comes from the rent range above. Figures are rounded to the nearest $10,000 and versioned as INCOME-CAP-2026-08b.
An appraiser may not agree with this number. Residential appraisals lead with comparable sales, not with income, and in neighborhoods without many permitted ADUs there may be few comparable sales to lead with. The known result is an "appraisal gap": the comps support less per square foot than the build costs. So treat the figure as what an income-minded buyer might pay for the unit — not as what a bank will lend against. If you plan to finance the build by borrowing against the added value, ask a local appraiser before you count on it.
Build cost is shown next to it on purpose. Detached ADU construction in Boston runs roughly $275–450 per square foot, which for most lots is the same order of magnitude as the value the ADU adds. Value added is not profit, and it is not money in hand — it's an estimate of what a rentable unit could be worth to the property, before you've paid to build it. Financing, condition, site constraints, and the permitting path all sit outside this arithmetic. It is a rough estimate, not an appraisal.
Housing capacity
The "space for" figure uses HUD's general guideline of about 2 persons per bedroom, applied to the same bedroom span as the rent estimate. It's a rough capacity figure, not a promise or a legal occupancy limit.
Boston & Massachusetts ADU resources
- City of Boston — Accessory Dwelling Units (ADUs) in Boston Program overview, ADU guidebook, design & budget workshops.
- City of Boston — ADU Financial Assistance Program Grants and loans for income-eligible homeowners to design, permit, and build an ADU.
- Boston Planning & Development Agency — ADU Zoning The City's ADU zoning rules in detail.
- Mass.gov — ADU Design Challenge Showcase Free, downloadable ADU plans from the state's 2026 design challenge, searchable by size and by features like accessibility and sustainable construction. Designs top out at 900 sq ft — the same ceiling as the backyard footprint this map estimates. Unlike the statewide law below, a plan is useful in Boston like anywhere else.
- Mass.gov — Accessory Dwelling Units The statewide ADU law (Affordable Homes Act, Chapter 150 of the Acts of 2024). Useful background — but it does not apply in Boston; see what Boston allows by right.
Buy me a coffee
This map is free to use and independently built and maintained. If it helped you, a coffee is always appreciated.
Buy me a coffee