What you can legally build on your lot, what it costs, and what it means for rental income or resale value.
California's aggressive ADU laws allow most homeowners to build an accessory dwelling unit, granny flat, or garage conversion up to 1,200 square feet on their property by-right, regardless of local zoning. Simi Valley has processed hundreds of ADU permits since 2020. Typical build costs range $180K to $350K for a detached ADU. Rental income typically adds $1,800 to $3,200 monthly in Simi Valley.
Starting in 2017 and expanded repeatedly since, California has systematically overridden local zoning to allow accessory dwelling units on most single-family and multi-family lots. Key provisions as of 2026:
Simi Valley has been processing ADU permits at volume since the state laws took effect. The city has a dedicated ADU planning process that runs more smoothly than some LA-area jurisdictions. Most Simi Valley lots, particularly older neighborhoods with larger flat lots, are well-suited to ADU construction.
| ADU Type | Typical Build Cost | Timeline |
|---|---|---|
| Garage conversion | $80K to $160K | 3 to 6 months |
| Attached ADU (600 to 900 sq ft) | $160K to $280K | 6 to 10 months |
| Detached ADU (600 to 1,200 sq ft) | $220K to $450K | 8 to 14 months |
| Prefab or modular ADU | $180K to $320K | 4 to 7 months |
An appraised ADU typically adds 70% to 100% of its build cost to the home's value, depending on quality, size, and rental potential. Legal ADUs with certificates of occupancy add more than unpermitted additions. For Simi Valley owners considering a build-and-hold rental income play, the math typically works over a 5 to 7 year horizon.
Most likely yes. California state law allows ADUs on nearly all single-family properties. Exact size and placement depend on lot dimensions, setbacks, and city requirements. Verify with Simi Valley's planning department for your specific parcel.
Garage conversions run $80K to $160K. New attached ADUs run $160K to $280K. Detached ADUs run $220K to $450K. Costs depend on size, finishes, and site challenges.
Generally no. California law prohibits HOAs from banning ADUs. They can set reasonable aesthetic standards but cannot block construction entirely.
Typical monthly rents for a 600 to 1,000 sq ft ADU in Simi Valley run $1,800 to $3,200. Location, quality of build, and whether the main house shares amenities affect the exact figure.
Yes. The ADU is assessed separately and added to your base. Under Prop 13, only the ADU's assessed value is added to your tax basis, not a reassessment of the full property.
Depends on Simi Valley's short-term rental rules. Currently, short-term rentals require specific permits and have operational restrictions. Long-term rentals (30+ days) are straightforward.
California law requires ministerial approval within 60 days of a complete application. In practice, Simi Valley often processes within 30 to 45 days. Construction timelines are separate.
Depends on priorities. Garage conversions are cheaper and faster but you lose parking and storage. Detached ADUs are more expensive but preserve the primary home's utility.
Yes. You need separate rental dwelling coverage. Expect premium increases of 10% to 30% depending on the ADU's size and use.
A licensed general contractor experienced with Simi Valley ADU permits is the right starting point. I can introduce you to local contractors who specialize in ADU builds.
Happy to help you evaluate your specific lot's ADU potential, introduce you to contractors, and run the ROI math.
Talk to Brian