The 10-Foot Separation Rule

The 10 feet that comes up on detached ADUs is a Building Code item, not a zoning one. Zoning’s accessory-building separation rule, LAMC 12.21 C.5(d), does not apply to an ADU: LAMC 12.22 A.33(c)(7) removes the passageway and space-between-buildings requirements for the ADU and for the other residential buildings on the lot (City Planning ADU memo, February 26, 2020, Q14). What plan check applies is fire separation under the Residential Code. Two buildings on one lot are checked as if a property line ran between them. An exterior wall within 5 feet of that assumed line needs one-hour construction, openings are not allowed within 3 feet and are limited to 25 percent of the wall from 3 to 5 feet, and projections such as eaves are not allowed within 2 feet (CRC R302.1 and Table R302.1(1)). Ten feet of separation lets both buildings sit 5 feet from the assumed line with no rated walls and no opening limits. That is where the 10 comes from. Closer than 10 feet is allowed; the design places the assumed line, rates the walls that fall within 5 feet of it, and limits the openings in them. Eaves count.

Zoning’s accessory-building separation rule, LAMC 12.21 C.5(d), does not apply to an ADU: LAMC 12.22 A.33(c)(7) removes the passageway and space-between-buildings requirements for the ADU and for the other residential buildings on the lot (City Planning ADU memo, February 26, 2020, Q14). PC/STR/Corr.Lst.20A Part III.D.1 and D.2 name the requirement on the ADU correction sheet for the applicable pathways.

The correction asks for 10 feet of separation or for rated construction measured from an assumed property line.

Treating the 10 feet as a zoning setback and moving the ADU, when the usual answer is a rated wall and a window schedule. The other mistake is forgetting the eaves: a wall can sit 5 feet from the assumed line while its overhang sits 3 feet from it.

ADU Pathway Selection (because this entry covers fire separation between buildings on one lot, and the pathway decides whether the ADU is a separate building), Yards and Setbacks (because rear-yard requirements interact with ADU placement), Lot Cut Date (because lots with multiple existing structures may have unresolved subdivision questions affecting siting), and the Encroachment Plane (where two-story ADUs trigger upper-floor setback rules independently).