21-23 Beacon St

21-23 Beacon St is a 10-story condominium building in Downtown Boston. Its floor plate has 83% of its area within daylight reach of a facade and scores 80 out of 100 on this screen’s measures, which is part of why it works as housing.

MA-7 & MA-8 · Screened 2026-10-04 · Already housing

Where 21-23 Beacon St is

Its block in Downtown Boston, drawn from Boston's own building outlines and street centerlines.

Suitability score

0–29

30–49

50–69

70–100

Already housing
Not screened
21-23 Beacon St

21-23 Beacon St and the 1,000 ft of Downtown Boston around it. The streets are drawn at the width of the right-of-way they run down, sidewalks included, and named, and every building on these blocks is drawn. What is left open and stippled is neither: a surface lot, a plaza or a vacant parcel. The screened buildings are colored by the ramp above, the same one the district map uses, carry their score, and each is a link to its own page. Each casts a shadow as long as its measured height lets it, so towers stand out from the low buildings around them; the unscreened buildings have no measured height and cast none. 21-23 Beacon St is outlined in its own score color at the outline it covers at ground level. Dashed inside it is the typical floor, the one the floor-plate figure below draws, because this building steps back as it rises. Its windowless core is hatched. The inset places it among every building the screen covers.

Typical floor plan: 83% of the building’s floor area sits within daylight reach of a facade; the drawing shows its typical floor, where the share is 83%. The shading is daylight, darkest at the windows and gone 35 ft in. The hatched area is windowless core: 2,808 sq ft per typical floor that must be renovated but can never be rented as an apartment. The dimension marks the deepest interior point, 57 ft from the nearest window. A–A is where the section beside it is cut.

Section A–A: cut across the depth of the plate, through the middle of the core. Daylight reaches 35 ft in from every facade, fading as it goes, and the hatched stripe left between the lit edges is floor that gets converted and can never be rented: 2,808 sq ft on the floor the screen scores, and 22,467 sq ft over 9 residential floors. The steps are where the building sets back as it rises. The shaded ground floor is converted like the rest and counted as no homes.

This is already a condominium building: Boston's assessing records list 155 condominiums here. It is on the map because a record on the same parcel carries an office land-use code, but this site does not count it as a conversion candidate.

Across its 9 residential floors this building carries 22,467 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 21-23 Beacon St, 2026-10-04.

Suitability scoreStrong candidate	80 / 100
Storiesfrom 120 ft aerial-survey height ÷ 12 ft floor-to-floor	10
Height bandsfloor plate by floor range, from the city's roof outlines	Floors 1–9: 16,641 sq ft · Floor 10: 1,395 sq ft
Floor platetypical floor, from the city outlines	16,641 sq ft
Daylight efficiencyshare within 35 ft of a facade	83%
Deepest interior pointviability threshold is 80 ft	57 ft
Windowless coretypical floor; the building-wide total is in the text above	2,808 sq ft
BuiltBoston's own record of the construction year	1999
ZoningDowntown Boston	PEMBERTON SQUARE PROTECTION AREA
Opportunity Zonenot in a 2018 designated tract	No

The plan: window line, shape and floor rhythm

39 ft of exterior wall for each home the screen estimates — enough for two- and three-bedroom homes across the plan.

The typical floor has 565 ft of exterior wall, and across 9 residential floors the building carries 4,690 ft of it in total, divided among the 119 homes this screen estimates.

Its plate is moderately compact at 0.65: 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 13.2 homes per floor, repeated on every residential floor, since this building's outline does not change as it rises.

Why 21-23 Beacon St scores 80 out of 100

Daylight efficiency 87, Interior depth 58, Scale 79, Finance 100 — each out of 100, then weighted.

Score components for 21-23 Beacon St. Points are the subscore times its weight.

Component	Subscore	Weight	Points
Daylight efficiency83% of the floor area within 35 ft of a facade, scored from 40% up to 90%	87	45%	39.0
Interior depthdeepest interior point 57 ft from a facade, scored from 80 ft down to 40 ft	58	25%	14.5
Scale119 estimated homes, scored from 0 up to 150	79	15%	11.9
Financecompleted value covers conversion cost with 23% 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 interior depth, at 58 out of 100: deepest interior point 57 ft from a facade, scored from 80 ft down to 40 ft.

Built in 1999

Boston records this building as going up in 1999, which puts it in the modern stock.

As a rule, newer systems and better structure, but usually built deep and sealed for office use, and rarely cheap enough to buy at a price that lets a conversion pencil. 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 83% daylight figure above deserves.

Other buildings screened near 21-23 Beacon 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.

12-14A Beacon St
9 stories · 100% daylit · 85 estimated homes · scores 94/100 (Strong candidate)
about 190 ft away
20 Ashburton Pl
8 stories · 99% daylit · 78 estimated homes · scores 93/100 (Strong candidate)
about 220 ft away
6-6B Beacon St
10 stories · 100% daylit · 92 estimated homes · scores 94/100 (Strong candidate)
about 250 ft away
69-81 Tremont St
14 stories · 72% daylit · 258 estimated homes · scores 64/100 (Worth a feasibility study)
about 410 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.