Accessory Dwelling Unit Guide

• Pros: Can increase property value by up to 30% and provides immediate rental income or space for multi-generational living.

• Cons: High initial investment, potential increase in property taxes, and the added responsibility of being a landlord if you choose to rent it out.

• Garage Conversions: This is often the cheapest route (saving 30-50%) because the foundation, walls, and roof already exist.

• Value Engineering: Work with your architect to use standard window sizes and “contractor-grade” finishes instead of custom luxury materials.

• Standardized Plans: Use city-provided “master plans” to bypass expensive design phases.

Since ADUs can be expensive, many homeowners use these options:

• HELOC or Home Equity Loan: Borrowing against your home’s current value.

• Cash-Out Refinance: Replacing your mortgage with a larger one and taking the difference in cash.

• Renovation Loans (FHA 203k or Fannie Mae HomeStyle): These allow you to borrow based on the future value of the home after the ADU is built.

• New Rule for 2026: Some lenders now allow you to use 75% of the future rental income from the ADU to help you qualify for the loan.

• Attached: Generally cheaper because you can utilize existing walls and often “piggyback” off the main home’s HVAC and plumbing systems.

• Detached: More expensive as it requires its own foundation, independent utility connections, and four new exterior walls. However, detached units often command higher rental income due to increased privacy.

Yes, significantly. A simple “dry” room like a bedroom or home office is the most affordable (averaging $80–$200 per square foot). “Wet” rooms like kitchens and bathrooms are the most expensive (reaching $250–$500+ per square foot) due to the cost of plumbing, waterproofing, venting, and expensive fixtures.

Your location determines what you can build. Zoning laws dictate “setbacks” (how far the unit must be from property lines), height limits, and parking requirements. Some cities have “pre-approved” plans that can save you thousands in architectural fees and speed up the permitting process, which otherwise might take months.

Labor typically accounts for about 40% of your total project cost. You can expect the following professional rates:

• General Contractor: 10% – 20% of the total project cost.

• Architect: 5% – 20% of the project cost (or $125–$250 per hour).

• Electrician/Plumber: $50 – $200 per hour.

Size is a primary cost driver, usually calculated at $150 to $350+ per square foot. Interestingly, the cost per square foot often decreases as the unit gets larger because fixed costs (like permits and utility hookups) are spread over more space. A small 200-square-foot unit might cost $400 per square foot, while a 1,000-square-foot unit might drop to $200 per square foot.

Costs vary widely based on the type of construction. As of 2026, a standard home addition typically ranges from $22,000 to over $83,000, with an average of around $51,000. However, building a full detached ADU is more expensive, with averages closer to $180,000, though high-end custom units can reach $360,000.

An ADU, often called a “granny flat” or “in-law suite,” is a secondary housing unit on the same lot as a primary single-family home. To be considered an ADU, the space must include independent facilities for living, sleeping, cooking, and sanitation. It can be detached (a small cottage), attached (an addition), or a conversion (like a garage or basement apartment).