Attached ADUs: more living space joined to the house

The old 50 percent cap and what replaced it

Ordinance 1349 limited an attached ADU built as an addition to 50 percent of the existing house or 800 square feet, whichever was larger, up to 1,000 square feet. On a 1,400 square foot ranch house in Rancho Simi that meant 800 square feet at most. HCD's October 29, 2025 findings faulted the section for not covering attached units on lots with new or multifamily dwellings and directed the City to rewrite it.

Separately, Government Code section 66321 says a city cannot cap an ADU below 850 square feet, or 1,000 square feet with more than one bedroom. Ordinance 1373 (reported adopted July 2026) is the City's response. Read its current size table before you size the unit; the 50 percent figure should not be your starting point.

Height, entrances and outside stairs

State law lets an attached ADU reach 25 feet or the height limit for the main house, whichever is lower. That is enough for a second-story unit over an attached garage or a family room, if the structure below can carry it.

HCD flagged two Simi Valley rules that shaped attached designs. The ordinance said the ADU's entrance should not face the street 'if possible', which HCD called a subjective standard. And it applied the City's setbacks for outside stairs (18 feet rear, 8 feet side) to stairs serving a second-story ADU, which HCD said exceeded state law because required stairs count as part of the ADU.

What changes in the main house

Adding an attached unit does not, by itself, require you to add fire sprinklers to the existing house; state law says ADU construction cannot trigger that. The shared wall still needs rated construction, and the new unit needs its own exterior access.

• Plan the electrical service early: two dwellings on one service panel may need an upgrade.
• Confirm where the new bathroom drains tie in before fixing the floor plan.
• Keep the ADU's entrance and the house's front door clearly separate on the site plan.