309 Callowhill St
309 Callowhill St is a 6-story building in Center City Philadelphia with 76% of its floor area within daylight reach of a facade. This screen estimates 146 homes and scores it 62 out of 100 for office-to-residential conversion suitability.
PA-2 & PA-3 · Screened 2026-09-15 · Worth a feasibility study
No address point in Philadelphia’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 309 Callowhill St is
Its block in Center City Philadelphia, drawn from Philadelphia's own building outlines and street centerlines.
- Already housing
- Not screened
- 309 Callowhill St
This building does not pencil at default assumptions, even with a modeled RESIDE Act grant.
Every floor carries 8,914 sq ft of windowless core — area that must be gutted, re-plumbed and sprinklered but can never be rented as an apartment. Across 5 residential floors that is 44,568 sq ft of cost carrying no revenue.
Modeled conversion metrics for 309 Callowhill St, 2026-09-15.
| Suitability scoreWorth a feasibility study | 62 / 100 |
|---|---|
| Storiesfrom 90 ft LiDAR height ÷ 12 ft floor-to-floor | 6 |
| Floor plateper floor, from the city footprint | 36,441 sq ft |
| Daylight efficiencyshare within 35 ft of a facade | 76% |
| Deepest interior pointviability threshold is 80 ft | 63 ft |
| Windowless coreper floor — renovated but never rentable | 8,914 sq ft |
| Estimated homesat 750 sq ft average | 146 |
| Conversion cost$360/sq ft all-in on 182,203 sq ft | $65.6M |
| Gap before grantshortfall against completed value | $36.9M |
| Modeled RESIDE grantbelow the statutory cap | $8.8M |
| BuiltPhiladelphia's own record of the construction year | 1939 |
| ZoningCenter City Philadelphia | CMX3 |
| Opportunity Zonenot in a 2018 designated tract | No |
The plan: window line, shape and floor rhythm
36 ft of exterior wall for each home the screen estimates — enough for two- and three-bedroom homes across the plan.
The typical floor has 1,063 ft of exterior wall, and across 5 residential floors the building carries 5,317 ft of it in total, divided among the 146 homes this screen estimates.
Its plate is elongated at 0.41: a slab or an L, which is why so much of the floor lies within reach of a window: the outline buys daylight that a square plan of the same area would not. How window line and plate shape are measured.
At the model’s 750 sq ft average, the plan works out at about 29.2 homes per floor, repeated on every residential floor, since this building's outline does not change as it rises.
Why 309 Callowhill St scores 62 out of 100
Daylight efficiency 71, Interior depth 43, Scale 97, Finance 29 — each out of 100, then weighted.
Score components for 309 Callowhill St. Points are the subscore times its weight.
| Component | Subscore | Weight | Points |
|---|---|---|---|
| Daylight efficiency76% of the floor area within 35 ft of a facade, scored from 40% up to 90% | 71 | 45% | 32.0 |
| Interior depthdeepest interior point 63 ft from a facade, scored from 80 ft down to 40 ft | 43 | 25% | 10.9 |
| Scale146 estimated homes, scored from 0 up to 150 | 97 | 15% | 14.6 |
| Financeremaining gap is 43% of conversion cost, scored from 60% down to 0% | 29 | 15% | 4.3 |
Most of what it earns comes from daylight efficiency, and what holds it back most is finance, at 29 out of 100: remaining gap is 43% of conversion cost, scored from 60% down to 0%.
Built in 1939
Philadelphia records this building as going up in 1939, which puts it in the interwar stock.
As a rule, steel or concrete frames with punched windows and plates still narrow enough for daylight to reach the middle, often with high ceilings. Among the most convertible stock there is, and the reason so many early conversions were of buildings this age. 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 76% daylight figure above deserves.
What the conversion would cost
$360 per square foot all-in across 182,203 sq ft of converted floor, or $65.6M in total — about $449K for each of the 146 homes.
All-in cost per square foot for 309 Callowhill 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 44,568 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 $197K a home, which for 146 homes is $28.7M.
What would have to be true
A $36.9M gap before any grant, $28.1M after the modeled one. The smallest single change that would close it: average unit size 48% down.
The shortfall before any grant is $36.9M. Closing it from the value side means each of the 146 homes finishing at $449K rather than the $197K the model assumes — 128% more. Closing it from the cost side means getting all-in cost down from $360 to $158 per square foot.
A modeled RESIDE grant of $8.8M moves that threshold to $389K a home. That is still above the $197K assumed, leaving $28.1M uncovered, which is why the grant does not unlock this one.
Nor do all four levers have to move. For 309 Callowhill St, one alone would close the gap at:
Single-assumption flip points for 309 Callowhill St, each holding the other three at their defaults.
| Average unit sizethe binding constraint — the least this building would need | 48% downto 389 sq ft |
|---|---|
| Conversion cost per square foot | 59% downto $106/sq ft |
| Completed value per home | 98% upto $389K |
All four are sliders in the survey, where 309 Callowhill St re-scores against your own numbers.
How it compares in Center City Philadelphia
309 Callowhill St ranks 56th of the 103 buildings screened here, against a median score of 67.
Its floor plate is better lit than 45% of them, and shallower than 41% of them at its deepest interior point. On size it is the 59th largest by estimated homes. The full survey of all 103 buildings ranks and maps every one of them, and the list by address links each to its own page.
Within the CMX3 zone
10th of 14 zoned CMX3 — the comparison that shares its rules, rents and streets.
14 of the screened buildings are zoned CMX3. 309 Callowhill St ranks 10th of them against a median score of 73. 8 score 70 or more, their median deepest interior point is 53 ft from a facade against this building’s 63 ft, and together they would hold an estimated 1,122 homes. The group’s top scorer is 461 N 3rd St, at 84.
Its nearest match in size: 801 Arch St
The screened building closest to 309 Callowhill St in converted floor area is 801 Arch St: 176,095 sq ft over 6 stories, against 182,203 sq ft over 6 here.
309 Callowhill St scores 62 to its 56, and with the size held roughly equal most of that lead comes from daylight efficiency: 71 out of 100 here, 62 there (71% of the floor area within 35 ft of a facade, scored from 40% up to 90%). On the money they sit in the same place: each falls short even with the modeled grant. The two stand about 2,900 ft apart.
Would it qualify for a RESIDE Act grant?
The model awards 309 Callowhill St $8.8M, 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 309 Callowhill St
The 4 closest the screen also covers, from 520 ft away. They are the colored outlines on the plan at the top of this page.
- 461 N 3rd St5 stories · 100% daylit · 53 estimated homes · scores 84/100 (Strong candidate)about 520 ft away
- 441 N 5th St4 stories · 73% daylit · 48 estimated homes · scores 46/100 (Marginal)about 530 ft away
- 421 N 7th St7 stories · 85% daylit · 229 estimated homes · scores 79/100 (Strong candidate)about 1,350 ft away
- 417 N 8th St5 stories · 85% daylit · 103 estimated homes · scores 72/100 (Strong candidate)about 1,770 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.