How much does an ADU cost?

Basement conversion: $25,000-$75,000. Garage conversion: $30,000-$100,000. Attached addition: $50,000-$200,000. Detached new: $100,000-$300,000+. Prefab/modular: $50,000-$200,000. Per sq ft: conversions: $50-$150. New construction: $150-$400. Most affordable: garage or basement conversion (uses existing structure). Additional costs: permits ($1,000-$10,000+). Plans ($2,000-$15,000). Utility connections ($2,000-$10,000). Impact fees ($2,000-$20,000+ in some areas).

Are ADUs legal in my area?

ADU laws have changed dramatically. Many states now require cities to allow ADUs: California (SB 9, AB 2221), Oregon, Washington, Vermont, Connecticut, and more. How to check: call your local planning department. Search your city/county code for ADU or accessory dwelling unit. Check your state legislature website for recent ADU legislation. Key questions: is an ADU allowed on my property? What are the size limits? What are the setback requirements? Do I need to live on the property? Are there parking requirements? What permits and fees are required? Trend: most cities are making ADUs easier to build. Many have eliminated barriers like parking requirements and owner-occupancy mandates.

How much rental income can an ADU generate?

Average monthly rent: studio/1-bed ADU: $800-$1,800. 2-bed ADU: $1,200-$2,500. Varies greatly by location (city vs suburbs, rental market). Annual income: $10,000-$30,000+. Short-term rental (Airbnb): potentially 30-50% more than long-term rental. More management and turnover. Check local short-term rental regulations. ROI: 5-10% annual return on investment. Payback: 5-15 years. Can offset or cover mortgage payment. Tax benefits: depreciation, expense deductions (consult a tax professional). Factors: location and local rental market. ADU size and quality. Parking availability. Separate entrance and outdoor space. Kitchen quality (full kitchen commands higher rent).

How long does it take to build an ADU?

Planning and permits: 2-6 months (varies greatly by jurisdiction). Construction: garage conversion: 2-3 months. Basement conversion: 2-4 months. Attached addition: 3-6 months. Detached new construction: 4-8 months. Prefab/modular: 4-8 weeks on-site (plus factory time). Total (start to finish): conversions: 4-8 months. New construction: 6-14 months. Prefab: 4-8 months (including permits). What takes longest: permit approval (the biggest variable). Some cities have streamlined ADU permits to 30-60 days. Others take 3-6 months.

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Woodworking & Lumber

ADU & In-Law Suite Guide: Types, Cost & Regulations

Lumberama Editorial Team, ADU & Accessory Dwelling Specialists Nov 23, 2026 9 min read

ADU Types

Detached ADU: Separate structure in the backyard. Most independent and private. Cost: $100,000-$300,000+. Sizes: 400-1,200 sq ft. Best for: rental income, maximum privacy.

Attached ADU: Addition to the existing house with separate entrance. Cost: $50,000-$200,000. Shares a wall with the main home. Best for: family members who want proximity but independence.

Garage conversion: Convert an existing garage to living space. Cost: $30,000-$100,000. Most affordable since the structure exists. Lose parking space.

Basement apartment: Convert finished or unfinished basement. Cost: $25,000-$75,000. Good headroom required (7 ft minimum for habitable space). Egress windows required (safety).

Above-garage apartment: Build living space above the garage. Cost: $80,000-$200,000. May require structural reinforcement.

Prefab/modular ADU: Factory-built units delivered and installed. Cost: $50,000-$200,000 (including site work). Faster construction (4-8 weeks on-site vs months). Brands: Abodu, Boxabl, Villa, Dvele.

Cost

By type: Basement conversion: $25,000-$75,000. Garage conversion: $30,000-$100,000. Attached addition: $50,000-$200,000. Detached new construction: $100,000-$300,000+. Prefab/modular: $50,000-$200,000 (turnkey).

Per square foot: Conversion (garage, basement): $50-$150/sq ft. New construction: $150-$400/sq ft. Prefab: $100-$300/sq ft.

Additional costs: Permits: $1,000-$10,000+. Architectural plans: $2,000-$15,000. Utility connections: $2,000-$10,000. Landscaping restoration: $1,000-$5,000. Impact fees (some jurisdictions): $2,000-$20,000+.

Regulations & Income

Zoning regulations: ADU laws have been significantly relaxed in many states (California, Oregon, Washington, etc.). Check local zoning: allowed by right vs conditional use. Size limits (often 800-1,200 sq ft or percentage of lot). Setback requirements. Parking requirements (many cities have eliminated these). Owner-occupancy requirements (some cities require owner to live in one unit).

Rental income potential: Average ADU rental: $800-$2,500/month depending on location and size. Can cover mortgage payments. Short-term rental (Airbnb): potentially higher income but more management. Long-term rental: steady income, less management.

ROI: ADUs can increase property value by 20-35%. Rental income can provide 5-10% annual return on investment. Payback period: 5-15 years depending on construction cost and rental market.

Other uses: Home office or studio. Guest house. Aging-in-place (for elderly parents). Adult children returning home. Home-based business.