A home addition cost calculator provides realistic cost estimates for adding square footage to your home, whether that is a new room, garage, ADU, or sunroom.
Estimate Addition Cost
How to Estimate Home Addition Costs
This home addition cost calculator provides planning estimates for the most common types of home additions. Cost per square foot varies significantly by addition type, foundation, and finishes.
Why Foundation Type Matters
A full basement adds $20-$40/sqft compared to a slab foundation. For a 400 sqft addition, this is $8,000-$16,000 extra. A crawl space falls in the middle. If you are adding onto an existing structure, your contractor may match the existing foundation type for structural continuity.
ADU Cost Considerations
ADUs are the most complex additions because they require separate utilities, their own HVAC, a full bathroom and kitchen. A garage conversion is the most cost-effective ADU approach at $50,000-$120,000. A detached ADU with full utilities typically costs $150,000-$350,000 depending on size and finishes.
Getting Accurate Quotes
Get at least 3 quotes from licensed contractors. Ensure quotes include all work: foundation, framing, roofing, exterior finishes, insulation, drywall, electrical, plumbing, HVAC, and interior finishes. A good quote itemizes each category so you can compare line by line.
Frequently Asked Questions
How much does a room addition cost per square foot?
Room additions cost $100-$300 per square foot for mid-range work. Budget additions run $75-$150/sqft. Luxury additions with high-end finishes can reach $400-$600/sqft. These figures include structural work, finishes, electrical, and HVAC extension but exclude permits and design fees.
How much does an ADU cost to build?
Accessory Dwelling Units (ADUs) cost $150,000-$350,000 for a typical 600-800 square foot unit. Garage conversions are less expensive ($50,000-$100,000) since the structure exists. Detached ADUs with full utilities cost more. Costs vary significantly by region — California ADUs often run $250,000+.
What is the ROI on a home addition?
Most home additions recover 50-65% of cost in resale value. Adding a bedroom or bathroom typically has the best ROI. Sunrooms and four-season rooms recover 50-60%. ADUs have strong ROI in high-rent markets because they generate income and add substantial value. Location matters more than addition type.
Is this tool free?
Yes, completely free with no signup required. Toggle between square feet and square meters for international use.
How long does a home addition take?
A typical room addition takes 4-6 months from permit application to final inspection. Permitting alone can take 4-12 weeks in many jurisdictions. ADUs take 6-12 months. Sunrooms and garage additions are typically 2-4 months. Factor these timelines into your planning.
Do I need a permit for a home addition?
Yes, all structural additions require building permits in virtually every jurisdiction. Permits add cost ($1,000-$5,000+) and time but protect you legally and ensure the work meets code. Unpermitted additions create problems at resale and can trigger removal requirements.