421 N 7th St
421 N 7th St is a 7-story building in Center City Philadelphia with 85% of its floor area within daylight reach of a facade. This screen estimates 229 homes and scores it 79 out of 100 for office-to-residential conversion suitability.
PA-2 & PA-3 · Screened 2026-09-15 · Strong candidate
Where 421 N 7th St is
Its block in Center City Philadelphia, drawn from Philadelphia's own building outlines and street centerlines.
- Already housing
- Not screened
- 421 N 7th St
This building does not pencil at default assumptions, even with a modeled RESIDE Act grant.
Every floor carries 6,218 sq ft of windowless core — area that must be gutted, re-plumbed and sprinklered but can never be rented as an apartment. Across 6 residential floors that is 37,308 sq ft of cost carrying no revenue.
Modeled conversion metrics for 421 N 7th St, 2026-09-15.
| Suitability scoreStrong candidate | 79 / 100 |
|---|---|
| Storiesfrom 120 ft LiDAR height ÷ 12 ft floor-to-floor | 7 |
| Floor plateper floor, from the city footprint | 42,082 sq ft |
| Daylight efficiencyshare within 35 ft of a facade | 85% |
| Deepest interior pointviability threshold is 80 ft | 51 ft |
| Windowless coreper floor — renovated but never rentable | 6,218 sq ft |
| Estimated homesat 750 sq ft average | 229 |
| Conversion cost$360/sq ft all-in on 252,495 sq ft | $90.9M |
| Gap before grantshortfall against completed value | $45.9M |
| Modeled RESIDE grantat the statutory per-award cap | $10.0M |
| BuiltPhiladelphia publishes construction years, but none matched this footprint | — |
| ZoningCenter City Philadelphia | CMX3 |
| Opportunity Zonecensus tract 42101037600, 2018 designation | Yes |
The plan: window line, shape and floor rhythm
33 ft of exterior wall for each home the screen estimates — enough for two- and three-bedroom homes across the plan.
The typical floor has 1,262 ft of exterior wall, and across 6 residential floors the building carries 7,571 ft of it in total, divided among the 229 homes this screen estimates.
Its plate is elongated at 0.33: 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 38.2 homes per floor, repeated on every residential floor, since this building's outline does not change as it rises.
Why 421 N 7th St scores 79 out of 100
Daylight efficiency 90, Interior depth 71, Scale 100, Finance 34 — each out of 100, then weighted.
Score components for 421 N 7th St. Points are the subscore times its weight.
| Component | Subscore | Weight | Points |
|---|---|---|---|
| Daylight efficiency85% of the floor area within 35 ft of a facade, scored from 40% up to 90% | 90 | 45% | 40.7 |
| Interior depthdeepest interior point 51 ft from a facade, scored from 80 ft down to 40 ft | 71 | 25% | 17.9 |
| Scale229 estimated homes, scored from 0 up to 150 | 100 | 15% | 15.0 |
| Financeremaining gap is 39% of conversion cost, scored from 60% down to 0% | 34 | 15% | 5.1 |
Most of what it earns comes from daylight efficiency, and what holds it back most is finance, at 34 out of 100: remaining gap is 39% of conversion cost, scored from 60% down to 0%.
What the conversion would cost
$360 per square foot all-in across 252,495 sq ft of converted floor, or $90.9M in total — about $397K for each of the 229 homes.
All-in cost per square foot for 421 N 7th 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 37,308 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 229 homes is $45.0M.
What would have to be true
A $45.9M gap before any grant, $35.9M after the modeled one. The smallest single change that would close it: average unit size 44% down.
The shortfall before any grant is $45.9M. Closing it from the value side means each of the 229 homes finishing at $397K rather than the $197K the model assumes — 102% more. Closing it from the cost side means getting all-in cost down from $360 to $178 per square foot.
A modeled RESIDE grant of $10.0M moves that threshold to $353K a home. That is still above the $197K assumed, leaving $35.9M uncovered, which is why the grant does not unlock this one.
Nor do all four levers have to move. For 421 N 7th St, one alone would close the gap at:
Single-assumption flip points for 421 N 7th St, each holding the other three at their defaults.
| Average unit sizethe binding constraint — the least this building would need | 44% downto 418 sq ft |
|---|---|
| Conversion cost per square foot | 55% downto $118/sq ft |
| Completed value per home | 80% upto $353K |
All four are sliders in the survey, where 421 N 7th St re-scores against your own numbers.
How it compares in Center City Philadelphia
421 N 7th St ranks 39th of the 103 buildings screened here, against a median score of 67.
Its floor plate is better lit than 58% of them, and shallower than 57% of them at its deepest interior point. On size it is the 50th 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
5th of 14 zoned CMX3 — the comparison that shares its rules, rents and streets.
14 of the screened buildings are zoned CMX3. 421 N 7th St ranks 5th 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 51 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: 642 N Broad St
The screened building closest to 421 N 7th St in converted floor area is 642 N Broad St: 254,191 sq ft over 6 stories, against 252,495 sq ft over 7 here.
421 N 7th St scores 79 and 642 N Broad St only 35, and the difference is a hard fail rather than a matter of degree: the lower one is capped at 35 because its deepest interior point is 93 ft from a facade, past the 80 ft threshold beyond which conversion requires carving a light well. On the money they sit in the same place: each falls short even with the modeled grant. The two stand about 4,000 ft apart.
Would it qualify for a RESIDE Act grant?
The model awards 421 N 7th St $10.0M, 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.
This building does sit in census tract 42101037600, a 2018 designated Opportunity Zone, which Section 201 lets HUD favor — one of the few statutory priorities this screen can actually check.
Other buildings screened near 421 N 7th St
The 4 closest the screen also covers, from 430 ft away. They are the colored outlines on the plan at the top of this page.
- 417 N 8th St5 stories · 85% daylit · 103 estimated homes · scores 72/100 (Strong candidate)about 430 ft away
- 441 N 5th St4 stories · 73% daylit · 48 estimated homes · scores 46/100 (Marginal)about 890 ft away
- 309 Callowhill St6 stories · 76% daylit · 146 estimated homes · scores 62/100 (Worth a feasibility study)about 1,350 ft away
- 990 Spring Garden St7 stories · 76% daylit · 109 estimated homes · scores 63/100 (Worth a feasibility study)about 1,460 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.