100 City Hall Plz
100 City Hall Plz is a 7-story building in Downtown Boston with 100% of its floor area within daylight reach of a facade. This screen estimates 58 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 100 City Hall Plz is
Its block in Downtown Boston, drawn from Boston's own building outlines and street centerlines.
- Already housing
- Not screened
- 100 City Hall Plz
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 100 City Hall Plz, 2026-10-04.
| Suitability scoreStrong candidate | 91 / 100 |
|---|---|
| Storiesfrom 80 ft aerial-survey height ÷ 12 ft floor-to-floor | 7 |
| Floor plateper floor, from the city footprint | 9,123 sq ft |
| Daylight efficiencyshare within 35 ft of a facade | 100% |
| Deepest interior pointviability threshold is 80 ft | 35 ft |
| Windowless corea measured sliver under 10 sq ft, not counted as windowless floor | 0 sq ft |
| Estimated homesat 750 sq ft average | 58 |
| Conversion cost$360/sq ft all-in on 54,738 sq ft | $19.7M |
| Gap before grantpencils unsubsidized | -$5.9M |
| Modeled RESIDE grantbelow the statutory cap | $3.5M |
| BuiltBoston's own record of the construction year | 1899 |
| ZoningDowntown Boston | SKY-LOW-DT |
| Opportunity Zonenot in a 2018 designated tract | No |
The plan: window line, shape and floor rhythm
50 ft of exterior wall for each home the screen estimates — enough for two- and three-bedroom homes across the plan.
The typical floor has 484 ft of exterior wall, and across 6 residential floors the building carries 2,904 ft of it in total, divided among the 58 homes this screen estimates.
Its plate is moderately compact at 0.49: 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 9.7 homes per floor, repeated on every residential floor, since this building's outline does not change as it rises.
Why 100 City Hall Plz scores 91 out of 100
Daylight efficiency 100, Interior depth 100, Scale 39, Finance 100 — each out of 100, then weighted.
Score components for 100 City Hall Plz. 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 35 ft from a facade, scored from 80 ft down to 40 ft | 100 | 25% | 25.0 |
| Scale58 estimated homes, scored from 0 up to 150 | 39 | 15% | 5.8 |
| Financecompleted value covers conversion cost with 48% 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 39 out of 100: 58 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 54,738 sq ft of converted floor, or $19.7M in total — about $340K for each of the 58 homes.
All-in cost per square foot for 100 City Hall Plz: 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 circulation and service area that earns nothing directly. Against it the model sets a completed value of $442K a home, which for 58 homes is $25.6M.
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 $19.7M of work with $5.9M to spare. Value per home would have to fall to $340K — 23% below the assumption — before the conversion stopped penciling, or all-in cost would have to rise from $360 to $468 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 100 City Hall Plz:
Single-assumption flip points for 100 City Hall Plz, 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 | 37% downto $280K |
| Average unit size | 46% upto 1,095 sq ft |
| Conversion cost per square foot | 66% upto $431/sq ft |
All four are sliders in the survey, where 100 City Hall Plz re-scores against your own numbers.
How it compares in Downtown Boston
100 City Hall Plz ranks 31st of the 96 buildings screened here, against a median score of 89.
Its floor plate is better lit than 82% of them, and shallower than 81% of them at its deepest interior point. On size it is the 84th 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
14th 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. 100 City Hall Plz ranks 14th 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 35 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: 70 Federal St
The screened building closest to 100 City Hall Plz in converted floor area is 70 Federal St: 56,476 sq ft over 9 stories, against 54,738 sq ft over 7 here.
The two land within a few points of each other — 91 here, 91 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 1,500 ft apart.
Would it qualify for a RESIDE Act grant?
The model awards 100 City Hall Plz $3.5M, 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 100 City Hall Plz
The 4 closest the screen also covers, from 90 ft away. They are the colored outlines on the plan at the top of this page.
- 25-55 Court St5 stories · 100% daylit · 46 estimated homes · scores 90/100 (Strong candidate)about 90 ft away
- 1-11 Court St16 stories · 85% daylit · 233 estimated homes · scores 90/100 (Strong candidate)about 230 ft away
- 28 State St42 stories · 83% daylit · 621 estimated homes · scores 88/100 (Strong candidate)about 360 ft away
- 30-40 Court St7 stories · 94% daylit · 290 estimated homes · scores 92/100 (Strong candidate)about 380 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.