Turning your garage into a legal ADU

Why state law favors garage conversions

A conversion within an existing structure is the most protected ADU category in Government Code section 66323(a)(1). It needs exterior access and setbacks for fire and safety, but not the 4-foot side and rear setbacks a new unit needs, which matters for the many detached garages built right on the rear property line.

• No replacement parking: the City cannot make you replace garage spaces lost to an ADU conversion (Gov Code 66314(d)(11)).
• Up to 150 square feet of expansion beyond an existing accessory structure for ingress and egress, such as a stair or entry.
• No fire sprinklers unless the main house is required to have them.
• No new sewer or water connection fee: an ADU built in existing space is not treated as a new residential use for connection fees unless built with a new house.

The 'garage is living area' problem HCD fixed

Ordinance 1349 counted garages as living area. State law (Government Code 66313) defines living area as interior habitable space, including basements and attics, but not a garage. HCD's October 29, 2025 findings told the City to remove garages from the definition, which affects how the size of your house, and so any size limit tied to it, is measured.

Ordinance 1349 also had a partial-garage rule for keeping some of the garage as parking. That rule belongs to the superseded ordinance; confirm what Ordinance 1373 says before you plan to split the space.

Legalizing a garage that was already converted

Plenty of Simi Valley garages were turned into bedrooms decades ago without a permit. State law now helps. Government Code 66322(b) bars the City from denying an ADU permit because of unpermitted structures or code violations that do not threaten health and safety and are not affected by the ADU work, and HCD told Simi Valley to say so in its ordinance.

For a unit built before January 1, 2020, Government Code 66311.7 goes further: the City cannot deny a permit for an unpermitted ADU because it violates building standards or ADU rules, unless correcting the violation is needed to protect health and safety or the building is substandard. Expect opened walls, a new electrical review and some rework; the structure still has to meet the building code.