Allowable area comes from 506.2.1, 506.2.2 and the frontage increase in 506.3, using Table 506.2 factors.
Unsprinklered (NS) factors, e.g. VB: B 9,000, M 9,000, A-2/A-3 6,000, E 9,500, R-1/R-2/R-4 7,000, S-1 9,000, U 5,500; R-3 is unlimited. For these groups the sprinklered one-story factor (S1) is 4 × NS and multistory (SM) is 3 × NS.
Per-story allowable area Aa = At + (NS × If). Above three stories, total area = [At + (NS × If)] × Sa, with Sa = 3 (4 with 13R); no single floor may exceed the per-story value.
Mixed buildings use 508.3.2 or 508.4.2 per story; above three stories the sum of each story's actual/allowable ratios may not exceed 3 (4 with 13R and separated occupancies).
The open space must be at least 20 ft wide, measured from the building to the nearest interior lot line, across the full street, or to another building on the lot; use the narrowest qualifying width.
Frontage increase If from Table 506.3.3: by % of perimeter (25–50, 50–75, 75–100) and open space width (20–25, 25–30, 30+ ft): 0.17/0.21/0.25, 0.33/0.42/0.50, 0.50/0.63/0.75; interpolation allowed.
Height, stories and area may not exceed Sections 504 and 506 for the construction type (602) and occupancy (302); each is checked separately, and each part separated by fire walls counts as its own building.
Searches all 4,276 IRC 2024 sections, 5,532 IBC 2024 sections and defined terms (Chapters 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33 so far) and each city's local rules as they are added. Section numbers and titles from the International Residential Code and International Building Code, 2024 editions, copyrighted by the International Code Council; the IRC's electrical chapters (34 to 43) follow NFPA 70, the National Electrical Code. Requirements are summarized in our words (IRC Chapters 3 to 43, IBC Chapters 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33); read the full text at the link on each result. Local values come from each city's published documents.
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