20 Custom House St
20 Custom House St is a 12-story building in Downtown Boston with 98% of its floor area within daylight reach of a facade. This screen estimates 119 homes and scores it 97 out of 100 for office-to-residential conversion suitability.
MA-7 & MA-8 · Screened 2026-10-04 · Strong candidate
No address point in Boston’s records falls inside this footprint, so it is named by the closest one that no other screened building claims — usually a site entrance, but check the outline on the plan below before relying on the street number.
Where 20 Custom House St is
Its block in Downtown Boston, drawn from Boston's own building outlines and street centerlines.
- Already housing
- Not screened
- 20 Custom House St
This building's conversion pencils at default assumptions without any subsidy.
Across its 11 residential floors this building carries 2,836 sq ft of windowless core — area that must be gutted, re-plumbed and sprinklered but can never be rented as an apartment.
Modeled conversion metrics for 20 Custom House St, 2026-10-04.
| Suitability scoreStrong candidate | 97 / 100 |
|---|---|
| Storiesfrom 141 ft aerial-survey height ÷ 12 ft floor-to-floor | 12 |
| Height bandsfloor plate by floor range, from the city's roof outlines | Floors 1–11: 11,275 sq ft · Floor 12: 2,413 sq ft |
| Floor platetypical floor, from the city outlines | 11,275 sq ft |
| Daylight efficiencyshare within 35 ft of a facade | 98% |
| Deepest interior pointviability threshold is 80 ft | 39 ft |
| Windowless coretypical floor; the building-wide total is in the text above | 284 sq ft |
| Estimated homesat 750 sq ft average | 119 |
| Conversion cost$360/sq ft all-in on 115,164 sq ft | $41.5M |
| Gap before grantpencils unsubsidized | -$11.1M |
| Modeled RESIDE grantbelow the statutory cap | $7.1M |
| 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
42 ft of exterior wall for each home the screen estimates — enough for two- and three-bedroom homes across the plan.
The typical floor has 481 ft of exterior wall, and across 11 residential floors the building carries 5,004 ft of it in total, divided among the 119 homes this screen estimates.
Its plate is moderately compact at 0.61: a squarish plan, with enough perimeter for a conventional double-loaded corridor. How window line and plate shape are measured.
At the model’s 750 sq ft average, the plan works out at about 10.8 homes per floor, repeated on every residential floor, since this building's outline does not change as it rises.
Why 20 Custom House St scores 97 out of 100
Daylight efficiency 100, Interior depth 100, Scale 79, Finance 100 — each out of 100, then weighted.
Score components for 20 Custom House St. Points are the subscore times its weight.
| Component | Subscore | Weight | Points |
|---|---|---|---|
| Daylight efficiency98% of the floor area within 35 ft of a facade, scored from 40% up to 90% | 100 | 45% | 45.0 |
| Interior depthdeepest interior point 39 ft from a facade, scored from 80 ft down to 40 ft | 100 | 25% | 25.0 |
| Scale119 estimated homes, scored from 0 up to 150 | 79 | 15% | 11.9 |
| Financecompleted value covers conversion cost with 44% 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 79 out of 100: 119 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 98% daylight figure above deserves.
What the conversion would cost
$360 per square foot all-in across 115,164 sq ft of converted floor, or $41.5M in total — about $348K for each of the 119 homes.
All-in cost per square foot for 20 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 2,836 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 119 homes is $52.5M.
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 $41.5M of work with $11.1M to spare. Value per home would have to fall to $348K — 21% below the assumption — before the conversion stopped penciling, or all-in cost would have to rise from $360 to $456 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 20 Custom House St:
Single-assumption flip points for 20 Custom House St, each holding the other three at their defaults.
| Circulation efficiencythe binding constraint — the least this building would need | 31% downto 55% |
|---|---|
| Completed value per home | 35% downto $288K |
| Average unit size | 44% upto 1,083 sq ft |
| Conversion cost per square foot | 61% upto $418/sq ft |
All four are sliders in the survey, where 20 Custom House St re-scores against your own numbers.
How it compares in Downtown Boston
20 Custom House St ranks 10th of the 96 buildings screened here, against a median score of 89.
Its floor plate is better lit than 72% of them, and shallower than 72% of them at its deepest interior point. On size it is the 58th 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
5th 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. 20 Custom House St ranks 5th 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 39 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: 43-45 Devonshire St
The screened building closest to 20 Custom House St in converted floor area is 43-45 Devonshire St: 109,677 sq ft over 13 stories, against 115,164 sq ft over 12 here.
The two land within a few points of each other — 97 here, 96 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 1,100 ft apart.
Would it qualify for a RESIDE Act grant?
The model awards 20 Custom House St $7.1M, 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 20 Custom House St
The 4 closest the screen also covers, from 190 ft away. They are the colored outlines on the plan at the top of this page.
- 36-44 Broad St10 stories · 96% daylit · 248 estimated homes · scores 100/100 (Strong candidate)about 190 ft away
- 21 Custom House St9 stories · 92% daylit · 82 estimated homes · scores 91/100 (Strong candidate)about 200 ft away
- 177 Milk St10 stories · 90% daylit · 96 estimated homes · scores 89/100 (Strong candidate)about 250 ft away
- 88 Broad St9 stories · 96% daylit · 65 estimated homes · scores 91/100 (Strong candidate)about 320 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.