LA Baseline Mansionization Ordinance — FAR Math (2026)
The Baseline Mansionization Ordinance (BMO), codified at LAMC §12.21.1-A.17 and adopted in 2008 then overhauled on March 17, 2017, caps floor area on LA single-family lots by a ratio to lot size. R1-1 zones cap at 0.45 FAR before bonuses. This guide walks the math lot-by-lot, explains the four exemptions that do not count toward FAR, shows how a 6,000 sqft plan on a 7,500 sqft lot fails while a 5,300 sqft plan passes, and covers the height, setback, and adjacent-ordinance stack that often surprises first-time homeowners on an addition or ground-up project in the LA flats.
The master FAR table — what each R1 suffix allows
R1-1 (most of flat-lot single-family LA): 0.45 FAR base. A 7,500 sqft lot caps at 3,375 sqft of counted floor area.
R1V1, R1V2, R1F, R1R: same 0.45 base with suffix-specific modifiers for variation zones adopted under Ordinance 184802 (2017).
RE9, RE11, RE15, RE20, RE40: 0.40 FAR base in the Residential Estate zones, tightening as lot size tier increases. A 20,000 sqft RE20 lot caps at 8,000 sqft.
RA (Rural Agricultural): 0.35 FAR base.
Hillside-suffixed zones (R1-H, RE-H) step the FAR down further under the Baseline Hillside Ordinance in LAMC §12.21-C.10.
What counts toward FAR — and the four exemptions
Counts: every square foot of enclosed conditioned space measured to the exterior face of perimeter walls, including stairs at each floor they occupy.
Exemption 1 — garages. A 400-sqft garage exemption applies on each residential parcel. Garage area above 400 sqft counts.
Exemption 2 — covered porches and patios. Up to 250 sqft of covered but unconditioned porch does not count if it is open on at least 50% of its perimeter.
Exemption 3 — basements. Finished space with more than 50% of its wall height below grade does not count toward FAR under LAMC §12.03 definition.
Exemption 4 — ADUs up to 800 sqft. Gov Code §65852.2(a)(1)(D)(i) preempts local FAR for ADUs at or below 800 sqft.
The +0.10 green bonus and the +0.20 affordable bonus
LAMC §12.22-C.3 grants +0.10 FAR for compliance with the LA Green Building Code (LAGBC) above baseline — typically achieved via LEED Silver certification or LAGBC-defined prescriptive measures such as cool roof, Energy Star appliances, drought-tolerant landscape, and 20% exceeds-Title-24 energy performance.
A separate +0.20 affordable housing bonus applies when one or more restricted-affordable units are recorded against the deed for 55 years. Rarely used on single-family remodels but significant on SB 9 duplex projects.
Bonuses stack up to a combined ceiling of +0.30, which on a 7,500 sqft R1-1 lot adds 2,250 sqft of allowed floor area — a meaningful unlock for families building above the base 3,375 sqft cap.
The 45-degree encroachment plane — second-story sculpting
BMO imposes a sloped encroachment plane rising at 45 degrees from each side property line beginning at a height of 20 feet. Any part of the structure that pierces the plane must be set back further from the lot line.
Designed to protect neighbor daylight and reduce the wall effect that drove the 2006 neighborhood backlash against speculative mansion builds in the flats.
The practical consequence: a second-story bedroom wing typically has to sit at least 5 feet inside the side yard setback, or the exterior wall has to fold back to stay under the plane.
Prevailing front-yard setback — the 500-foot rule
LAMC §12.07-C.1 requires the front yard setback to match the prevailing setback within 500 feet of the parcel on the same block face. If three nearby homes sit 25 feet back, the new home must sit 25 feet back.
Catches an expensive number of homeowners who plan a front-facing addition that protrudes past the established line. The fix is usually to push the addition into the side or rear yard — but the rear-yard buildable area may already be consumed by the ADU and pool, creating a three-way geometry fight.
The 2017 overhaul — what changed on March 17, 2017
Pre-2017 BMO allowed FAR bonuses for 3-car-garage orientations and articulated facades that builders routinely gamed. Ordinance 184802 eliminated those bonuses and replaced them with the green + affordable bonuses above.
The 2017 overhaul also tightened the garage exemption from 400 sqft per dwelling to a single 400 sqft total per parcel, closing a common duplex loophole.
Ordinance 184802 also tightened the covered-porch exemption: before 2017 a covered porch had to be open on just one side to qualify. The 2017 amendment requires the porch to be open on at least 50% of its perimeter, closing another geometry loophole.
If your remodel team is citing pre-2017 FAR rules, the design is wrong for 2026. Always confirm which BMO version the calculation uses.
Common design choices that waste FAR
Double-height entry foyers: any floor plate cutout is typically counted as the floor area on the level it belongs to. A two-story foyer does not double-count, but it also does not free up FAR on upper levels.
Oversized garages: the 400 sqft garage exemption applies only to the first 400 sqft. Garages above that count against primary-house FAR, so a 700 sqft 3-car garage consumes 300 sqft of the FAR budget.
Unfinished attic with habitability intent: attics shown as storage but accessible by pull-down ladder often end up inhabited post-permit. LADBS inspectors check stair access and insulation to classify attics. Finished attic counts fully toward FAR.
Mechanical penthouse or rooftop deck structures: these count toward height and sometimes toward FAR, depending on wall height and enclosed volume.
Careful early-stage FAR planning prevents redesigns mid-permit. A BMO worksheet should be part of the schematic design deliverable, not discovered during plan-check.
Adjacent ordinances that compound BMO — hillside, HPOZ, specific plans
BMO operates on top of other LA land-use ordinances rather than replacing them. A hillside parcel runs under BMO and under BHO (LAMC §12.21-C.10) simultaneously, with the more restrictive rule controlling each specific design metric.
HPOZ (Historic Preservation Overlay Zone) adds a design-review overlay on 37 city-of-LA neighborhoods. Inside an HPOZ, FAR might be unlimited under zoning but blocked by HPOZ incompatibility findings from the Historic Preservation Board.
Specific Plans govern select neighborhoods with their own development standards. The Hollywood Community Plan, Venice Specific Plan, and Cornfield Arroyo Specific Plan all impose their own FAR and height controls that override or modify BMO.
Community Design Overlays (CDOs) in neighborhoods like West Adams and Vermont Knolls set architectural-style requirements that narrow the BMO-compliant design envelope even further.
Reading zoning correctly in LA requires checking ZIMAS (Zoning Information and Map Access System, https://zimas.lacity.org) for the base zone, all overlays, specific plans, HPOZ status, and any supplementary design districts before committing to a scope.
Height, stories, and the residential floor-area-plus-height envelope
BMO limits building height in conjunction with FAR. R1-1 base zone allows 33 feet maximum to the highest point with pitched roofs allowed up to 36 feet under LAMC §12.21.1-A.10. Flat roofs cap at 30 feet under the same section.
Two-story residences are the norm in LA flats. Three-story homes are permitted only where lot area and zoning allow sufficient FAR absorption — typically lots over 10,000 sqft in R1-1.
The 45-degree encroachment plane compounds height limits. A wide box-style second story is often FAR-compliant on paper but physically impossible under the encroachment plane, forcing the design toward articulated massing with setbacks.
BHO (Baseline Hillside Ordinance) reduces allowable height on slope parcels by 5–10 feet depending on slope percentage, further shaping architecture on the hillside neighborhoods of Hollywood Hills, Silver Lake, Mount Washington, and Pacific Palisades.
Small-lot carve-outs — the 350-sqft additional allowance
Lots below 4,500 sqft in R1 zones receive a 350-sqft additional FAR allowance under LAMC §12.21.1-A.17.b. The provision preserves viable development on narrow hillside lots and legacy small parcels in older neighborhoods.
A 4,000 sqft R1-1 lot at 0.45 FAR caps at 1,800 sqft. The small-lot additional allowance raises the usable cap to 2,150 sqft, enough to build a reasonable single-family home with an attached garage.
Not stackable with the green or affordable bonuses on the same project. The small-lot allowance is intended as a floor, not a booster on top of already-generous FAR.
The provision is particularly common in Echo Park, Silver Lake, and Mount Washington where pre-war subdivision geometry created many sub-4,500 sqft lots.
Tying the math into an addition design
Real-world FAR audit: a 1,400 sqft existing 1950s ranch on a 7,000 sqft R1-1 lot. Base FAR: 0.45 x 7,000 = 3,150 sqft. Existing house + 400 sqft garage stays under cap with 1,750 sqft remaining. Owner wants a 900 sqft second-story addition plus a 500 sqft detached ADU.
Addition counts toward primary FAR: 1,400 + 900 = 2,300 sqft, well under the 3,150 cap. Garage counts: +400 sqft (400 sqft exempt, so 0 net). Detached ADU up to 800 sqft is fully exempt under state law.
Total built area on lot: 1,400 existing + 900 addition + 400 garage + 500 ADU = 3,200 sqft of structure. BMO-counted area: 2,300 sqft. The project passes BMO and fits within 2,300 / 3,150 = 73% of the base cap.
This kind of math is what the architect should show the homeowner at schematic design. If the addition pushes past the cap, the options are green bonus (needs LAGBC compliance), smaller addition, or converting some of the addition into ADU-eligible square footage.
See our room-additions pillar for FAR math worked against more LA lots: https://askbaily.com/room-additions-los-angeles
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