ADU & Garage Conversion Guide

ADU regulations vary enormously by municipality. Some cities allow ADUs by right (you just need a building permit), others require discretionary approval from a planning commission, and some don't allow them at all.

Key zoning questions to answer before doing anything else:

• Does your zoning district allow ADUs?
• What are the maximum size limits? (Common: 800–1,200 sqft)
• What are the setback requirements? (Distance from property lines)
• Is owner-occupancy required? (Some cities require the property owner to live on-site)
• How many ADUs can you have on one lot?
• Are there parking requirements?
• Are there design requirements (matching architecture, height limits)?

California's ADU revolution: California SB 9 and SB 10 significantly expanded ADU rights statewide. Most California single-family lots now allow both an ADU and a Junior ADU (JADU) by right, with streamlined permitting. If you're in California, the rules are much more permissive than most other states.

How to find your zoning rules: Contact your city's planning or zoning department. Many cities now have ADU-specific information pages on their websites. Start there before hiring an architect or contractor.

Permit requirements: A garage conversion to living space always requires a building permit. You're changing the use of the space, which triggers code requirements for habitable rooms — insulation, HVAC, electrical, egress, ceiling height.

Key conversion requirements:

• Egress window: At least one egress window meeting IRC minimums (20" wide × 24" high × 5.7 sqft net clear opening). The garage door opening is typically replaced with a wall containing a window and/or entry door.
• Insulation: Garage walls and ceiling must be insulated to meet energy code. The garage door wall gets framed, insulated, and finished. Existing walls may need insulation added.
• HVAC: Living space needs climate control. Options: extend existing HVAC ductwork (if capacity allows), add a mini-split unit (most common and often best), or install a wall heater/AC unit.
• Electrical panel capacity: A full ADU adds significant electrical load. Your main panel may need an upgrade to support the additional circuits. A sub-panel in the ADU is standard.
• Plumbing (if adding bathroom/kitchen): Running supply and drain lines to a garage depends on the garage's relationship to the existing plumbing stack and sewer line. Concrete slab? Trenching through the slab is required for drain lines.
• Floor: Garage floors are typically sloped for drainage. The floor may need leveling, and a subfloor system is usually installed over the concrete.

Cost drivers: Foundation work (detached), plumbing complexity, kitchen vs. no kitchen, finishes quality, local labor rates, and permit/impact fees. Some cities charge significant impact fees for new ADUs — check before budgeting.

• ADU-specific loans: Some lenders (Mosaic, Haus, and others) offer loans specifically designed for ADU construction. These may factor in the projected rental income when qualifying you, which can increase your borrowing capacity.
• HELOC: If you have sufficient equity, a Home Equity Line of Credit can fund a garage conversion or smaller ADU. Advantage: draw funds as needed during construction. Risk: variable rate, home is collateral.
• Construction loan: A short-term loan that covers building costs, then converts to a permanent mortgage or gets paid off when complete. More complex than a HELOC but designed for new construction.
• Fannie Mae HomeStyle: A renovation loan that allows you to borrow based on the after-renovation value of the property, including the ADU. Can be combined with your primary mortgage into a single loan.
• Cash-out refinance: If you have significant equity and current rates are reasonable, refinancing your primary mortgage for a higher amount and using the difference to fund the ADU.

Median ADU rental income: Varies enormously by location. In major California metros, 1-bedroom ADUs rent for $1,500–$2,500/month. In lower-cost-of-living areas, $800–$1,500/month is typical. Research comparable rentals in your area on Zillow, Apartments.com, or Craigslist.

Payback period calculation:

Property tax implications: Adding an ADU typically increases your property's assessed value and therefore your property taxes. The amount varies by jurisdiction. In California, Prop 13 limits reassessment to the value of the addition only — your main home's assessed value doesn't change.

Landlord obligations: Renting an ADU makes you a landlord. You're responsible for maintaining habitable conditions, complying with local landlord-tenant laws, carrying landlord insurance, and potentially registering the rental unit. Understand these obligations before building.