21 Custom House St
21 Custom House St is a 9-story building in Downtown Boston with 92% of its floor area within daylight reach of a facade. This screen estimates 82 homes and scores it 91 out of 100 for office-to-residential conversion suitability.
MA-7 & MA-8 · Screened 2026-10-04 · Strong candidate
Where 21 Custom House St is
Its block in Downtown Boston, drawn from Boston's own building outlines and street centerlines.
- Already housing
- Not screened
- 21 Custom House St
This building's conversion pencils at default assumptions without any subsidy.
Every floor carries 846 sq ft of windowless core — area that must be gutted, re-plumbed and sprinklered but can never be rented as an apartment. Across 8 residential floors that is 6,766 sq ft of cost carrying no revenue.
Modeled conversion metrics for 21 Custom House St, 2026-10-04.
| Suitability scoreStrong candidate | 91 / 100 |
|---|---|
| Storiesfrom 112 ft aerial-survey height ÷ 12 ft floor-to-floor | 9 |
| Floor plateper floor, from the city footprint | 10,459 sq ft |
| Daylight efficiencyshare within 35 ft of a facade | 92% |
| Deepest interior pointviability threshold is 80 ft | 43 ft |
| Windowless coreper floor — renovated but never rentable | 846 sq ft |
| Estimated homesat 750 sq ft average | 82 |
| Conversion cost$360/sq ft all-in on 83,675 sq ft | $30.1M |
| Gap before grantpencils unsubsidized | -$6.1M |
| Modeled RESIDE grantbelow the statutory cap | $4.9M |
| BuiltBoston's own record of the construction year | 1987 |
| ZoningDowntown Boston | SKY-LOW-DT |
| Opportunity Zonenot in a 2018 designated tract | No |
The plan: window line, shape and floor rhythm
40 ft of exterior wall for each home the screen estimates — enough for two- and three-bedroom homes across the plan.
The typical floor has 415 ft of exterior wall, and across 8 residential floors the building carries 3,317 ft of it in total, divided among the 82 homes this screen estimates.
Its plate is compact at 0.76: close to a square or a circle, the most efficient envelope an office can have and the least generous one for apartments: little perimeter to divide among homes. How window line and plate shape are measured.
At the model’s 750 sq ft average, the plan works out at about 10.3 homes per floor, repeated on every residential floor, since this building's outline does not change as it rises.
Why 21 Custom House St scores 91 out of 100
Daylight efficiency 100, Interior depth 92, Scale 55, Finance 100 — each out of 100, then weighted.
Score components for 21 Custom House St. Points are the subscore times its weight.
| Component | Subscore | Weight | Points |
|---|---|---|---|
| Daylight efficiency92% of the floor area within 35 ft of a facade, scored from 40% up to 90% | 100 | 45% | 45.0 |
| Interior depthdeepest interior point 43 ft from a facade, scored from 80 ft down to 40 ft | 92 | 25% | 23.1 |
| Scale82 estimated homes, scored from 0 up to 150 | 55 | 15% | 8.2 |
| Financecompleted value covers conversion cost with 37% to spare, which tops out the component | 100 | 15% | 15.0 |
Most of what it earns comes from daylight efficiency, and what holds it back most is scale, at 55 out of 100: 82 estimated homes, scored from 0 up to 150.
Built in 1987
Boston records this building as going up in 1987, which puts it in the late-modern stock.
As a rule, typically the hardest to convert: the deepest plates of any era, large central cores, and floor-to-floor heights tight enough that running residential drainage under a ceiling can be the thing that ends a project. That is the era’s general tendency rather than a finding about this building — the model reads an outline and a height, and can see nothing inside — but it is the first thing an architect would want to know, and it colours how much confidence the 92% daylight figure above deserves.
What the conversion would cost
$360 per square foot all-in across 83,675 sq ft of converted floor, or $30.1M in total — about $367K for each of the 82 homes.
All-in cost per square foot for 21 Custom House St: base cost plus the code work a change of use triggers.
| Base conversion cost | $260/sq ft |
|---|---|
| Plumbing risers to every unit | $40/sq ft |
| Operable windows and glazing | $30/sq ft |
| Additional egress stair | $18/sq ft |
| Fire sprinklers (NFPA 13R) | $12/sq ft |
| All-in | $360/sq ft |
That cost is charged across every converted square foot, including the 6,766 sq ft of windowless core, which has to be gutted, re-plumbed and sprinklered like the rest of the building and then earns nothing. Against it the model sets a completed value of $442K a home, which for 82 homes is $36.2M.
What would have to be true
This one already clears the bar. The question is how much room it has before it stops.
At $442K a home, completed value covers the $30.1M of work with $6.1M to spare. Value per home would have to fall to $367K — 17% below the assumption — before the conversion stopped penciling, or all-in cost would have to rise from $360 to $433 per square foot. Neither is a wide margin in a market that moves, but this building does not need a grant to work.
Each of these, moved on its own, would be enough to undo that for 21 Custom House St:
Single-assumption flip points for 21 Custom House St, each holding the other three at their defaults.
| Circulation efficiencythe binding constraint — the least this building would need | 26% downto 59% |
|---|---|
| Completed value per home | 30% downto $307K |
| Average unit size | 34% upto 1,009 sq ft |
| Conversion cost per square foot | 51% upto $391/sq ft |
All four are sliders in the survey, where 21 Custom House St re-scores against your own numbers.
How it compares in Downtown Boston
21 Custom House St ranks 27th of the 96 buildings screened here, against a median score of 89.
Its floor plate is better lit than 58% of them, and shallower than 64% of them at its deepest interior point. On size it is the 70th largest by estimated homes. The full survey of all 96 buildings ranks and maps every one of them, and the list by address links each to its own page.
Within the SKY-LOW-DT zone
12th of 25 zoned SKY-LOW-DT — the comparison that shares its rules, rents and streets.
25 of the screened buildings are zoned SKY-LOW-DT. 21 Custom House St ranks 12th of them against a median score of 91. 24 score 70 or more, their median deepest interior point is 35 ft from a facade against this building’s 43 ft, and together they would hold an estimated 2,629 homes. The group’s top scorer is 36-44 Broad St, at 100.
Its nearest match in size: 42-60 Federal St
The screened building closest to 21 Custom House St in converted floor area is 42-60 Federal St: 82,690 sq ft over 10 stories, against 83,675 sq ft over 9 here.
The two land within a few points of each other — 91 here, 89 there — so at this size the screen reads them as much the same prospect. On the money they sit in the same place: each pencils without a subsidy. The two stand about 1,200 ft apart.
Would it qualify for a RESIDE Act grant?
The model awards 21 Custom House St $4.9M, on the assumption it qualifies. Whether it does, no published dataset can say.
That figure assumes the building meets the Act’s vacant-and-abandoned test, which turns on code-enforcement and receivership records no city publishes.
It does not sit in a 2018 designated Opportunity Zone, so it would not draw that Section 201 priority.
Other buildings screened near 21 Custom House St
The 4 closest the screen also covers, from 140 ft away. They are the colored outlines on the plan at the top of this page.
- 177 Milk St10 stories · 90% daylit · 96 estimated homes · scores 89/100 (Strong candidate)about 140 ft away
- 88 Broad St9 stories · 96% daylit · 65 estimated homes · scores 91/100 (Strong candidate)about 180 ft away
- 20 Custom House St12 stories · 98% daylit · 119 estimated homes · scores 97/100 (Strong candidate)about 200 ft away
- 36-44 Broad St10 stories · 96% daylit · 248 estimated homes · scores 100/100 (Strong candidate)about 370 ft away
A screen, not a feasibility study, from outlines and heights rather than plans: the method and how to correct a figure. If you know this building’s interior, you know what this model cannot see.