An accessory dwelling unit is one of the few improvements that can pay for itself in Simi Valley, thanks to strong local rents — but the rules, costs, and tax treatment all matter before you build.
What state and local rules allow
California's ADU laws (Government Code §66310 and following) require cities, including Simi Valley, to permit ADUs on most residential lots, approve them ministerially within set timelines, and cap fees, setbacks, and parking requirements. Simi Valley's local ordinance controls specifics like maximum size, height, and design — check the current rules before you design. Statewide owner-occupancy mandates for new ADUs were suspended through January 1, 2025 and many jurisdictions still don't impose them.
Cost, rent, and ROI
A detached 600–800 sq ft ADU in Simi Valley typically runs about $180,000–$260,000 all-in in 2026 and rents around $2,400/month; garage conversions can be cheaper. Because rent is high relative to build cost here, a cash-funded ADU often returns 7–11% cash-on-cash. Run your specific numbers with the ADU rental ROI calculator or the ADU ROI (payback) calculator.
Taxes and resale value
Only the new ADU's assessed construction value is added to your property tax base (roughly 1.1–1.25% per year); your existing home keeps its Proposition 13 basis. On resale, appraisers increasingly value income-producing ADUs on the income they generate, often adding value near or above build cost. An ADU also creates multigenerational flexibility — housing a parent or adult child today, renting later.
Frequently Asked Questions
Can I build an ADU in Simi Valley?
Yes — California law requires Simi Valley to permit ADUs on most residential lots with ministerial approval. Confirm size, height, and design limits in the current local ordinance.
How much does an ADU cost in Simi Valley?
About $180,000–$260,000 all-in for a detached 600–800 sq ft unit in 2026; garage conversions can be less. Get contractor bids for your lot.
Will an ADU raise my property taxes?
Only partially — the new ADU's assessed value is added at roughly 1.1–1.25% per year. Your existing home keeps its Prop 13 base.
Can I rent out an ADU in Simi Valley?
Yes — long-term rentals are broadly permitted under state law. Short-term-rental rules vary; confirm the current local ordinance.
