20 Winthrop Sq
20 Winthrop Sq is a 6-story building in Downtown Boston with 100% of its floor area within daylight reach of a facade. This screen estimates 36 homes and scores it 89 out of 100 for office-to-residential conversion suitability.
MA-7 & MA-8 · Screened 2026-10-04 · Strong candidate
Where 20 Winthrop Sq is
Its block in Downtown Boston, drawn from Boston's own building outlines and street centerlines.
- Already housing
- Not screened
- 20 Winthrop Sq
This building's conversion pencils at default assumptions without any subsidy.
This footprint has no windowless core at all: every point on the floor plate is within 35 ft of a facade, which is unusual and is the main reason it scores as well as it does.
Modeled conversion metrics for 20 Winthrop Sq, 2026-10-04.
| Suitability scoreStrong candidate | 89 / 100 |
|---|---|
| Storiesfrom 71 ft aerial-survey height ÷ 12 ft floor-to-floor | 6 |
| Floor plateper floor, from the city footprint | 6,857 sq ft |
| Daylight efficiencyshare within 35 ft of a facade | 100% |
| Deepest interior pointviability threshold is 80 ft | 36 ft |
| Windowless corea measured sliver under 10 sq ft, not counted as windowless floor | 7 sq ft |
| Estimated homesat 750 sq ft average | 36 |
| Conversion cost$360/sq ft all-in on 34,285 sq ft | $12.3M |
| Gap before grantpencils unsubsidized | -$3.6M |
| Modeled RESIDE grantbelow the statutory cap | $2.2M |
| BuiltBoston's own record of the construction year | 1899 |
| ZoningDowntown Boston | SKY |
| Opportunity Zonenot in a 2018 designated tract | No |
The plan: window line, shape and floor rhythm
47 ft of exterior wall for each home the screen estimates — enough for two- and three-bedroom homes across the plan.
The typical floor has 340 ft of exterior wall, and across 5 residential floors the building carries 1,702 ft of it in total, divided among the 36 homes this screen estimates.
Its plate is compact at 0.74: 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 7.2 homes per floor, repeated on every residential floor, since this building's outline does not change as it rises.
Why 20 Winthrop Sq scores 89 out of 100
Daylight efficiency 100, Interior depth 100, Scale 24, Finance 100 — each out of 100, then weighted.
Score components for 20 Winthrop Sq. Points are the subscore times its weight.
| Component | Subscore | Weight | Points |
|---|---|---|---|
| Daylight efficiency100% of the floor area within 35 ft of a facade, scored from 40% up to 90% | 100 | 45% | 45.0 |
| Interior depthdeepest interior point 36 ft from a facade, scored from 80 ft down to 40 ft | 100 | 25% | 25.0 |
| Scale36 estimated homes, scored from 0 up to 150 | 24 | 15% | 3.6 |
| Financecompleted value covers conversion cost with 46% 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 24 out of 100: 36 estimated homes, scored from 0 up to 150.
Built in 1899
Boston records this building as going up in 1899, which puts it in the pre-1920 stock.
As a rule, the era that converts most readily: load-bearing masonry, small floor plates laid out before deep-plan offices were possible, operable windows and generous floor-to-floor heights. The usual obstacles are not daylight but hazardous materials, lifts and bringing structure up to current code. 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 100% daylight figure above deserves.
What the conversion would cost
$360 per square foot all-in across 34,285 sq ft of converted floor, or $12.3M in total — about $343K for each of the 36 homes.
All-in cost per square foot for 20 Winthrop Sq: 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 36 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 36 homes is $15.9M.
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 $12.3M of work with $3.6M to spare. Value per home would have to fall to $343K — 22% below the assumption — before the conversion stopped penciling, or all-in cost would have to rise from $360 to $464 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 Winthrop Sq:
Single-assumption flip points for 20 Winthrop Sq, each holding the other three at their defaults.
| Circulation efficiencythe binding constraint — the least this building would need | 32% downto 55% |
|---|---|
| Completed value per home | 36% downto $283K |
| Average unit size | 46% upto 1,096 sq ft |
| Conversion cost per square foot | 64% upto $427/sq ft |
All four are sliders in the survey, where 20 Winthrop Sq re-scores against your own numbers.
How it compares in Downtown Boston
20 Winthrop Sq ranks 48th of the 96 buildings screened here, against a median score of 89.
Its floor plate is better lit than 79% of them, and shallower than 77% of them at its deepest interior point. On size it is the 94th 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 zone
23rd of 52 zoned SKY — the comparison that shares its rules, rents and streets.
52 of the screened buildings are zoned SKY. 20 Winthrop Sq ranks 23rd of them against a median score of 88. 40 score 70 or more, their median deepest interior point is 51 ft from a facade against this building’s 36 ft, and together they would hold an estimated 19,019 homes. The group’s top scorer is 147 Milk St, at 100.
Its nearest match in size: 55-61 Batterymarch St
The screened building closest to 20 Winthrop Sq in converted floor area is 55-61 Batterymarch St: 36,384 sq ft over 7 stories, against 34,285 sq ft over 6 here.
The two land within a few points of each other — 89 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 1,300 ft apart.
Would it qualify for a RESIDE Act grant?
The model awards 20 Winthrop Sq $2.2M, 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 Winthrop Sq
The 4 closest the screen also covers, from 110 ft away. They are the colored outlines on the plan at the top of this page.
- 77 Franklin St11 stories · 100% daylit · 77 estimated homes · scores 93/100 (Strong candidate)about 110 ft away
- 75-101 Federal St21 stories · 89% daylit · 257 estimated homes · scores 96/100 (Strong candidate)about 200 ft away
- 64-74 Franklin St10 stories · 91% daylit · 94 estimated homes · scores 90/100 (Strong candidate)about 220 ft away
- 101 Federal St31 stories · 91% daylit · 569 estimated homes · scores 93/100 (Strong candidate)about 240 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.