Home Addition Cost Calculator
Estimate the cost of a home addition based on size, room type, foundation, and HVAC — materials, labor, and total cost breakdown.
Project Details
Typical bedroom addition 150-300 sq ft, in-law suite 500-1000 sq ft
Estimated Cost
Low
$18,767
Average
$34,362
High
$50,148
Cost Breakdown
Materials
| Item | Qty | Low | Mid | High |
|---|---|---|---|---|
| Concrete Slab Foundation | 300 sq ft | $2,400 | $4,500 | $6,000 |
| Framing (walls, ceiling, structure) | 300 sq ft | $3,600 | $5,400 | $7,500 |
| Roofing (match existing) | 300 sq ft | $1,500 | $3,000 | $4,500 |
| Exterior Siding (match existing) | 624 sq ft | $3,120 | $6,240 | $9,360 |
| Interior Finishes (drywall, paint, trim, flooring) | 300 sq ft | $4,500 | $8,400 | $12,000 |
| Subtotal | $15,120 | $27,540 | $39,360 |
Labor & Fees
| Item | Qty | Low | Mid | High |
|---|---|---|---|---|
| Electrical (wiring, outlets, fixtures) | 300 sq ft | $900 | $1,500 | $2,400 |
| Plumbing (hose bib / minimal hookup) | 1 job | $200 | $500 | $1,000 |
| HVAC — Extend Existing System | 1 job | $2,000 | $3,500 | $5,000 |
| Permits, Engineering & Fees | 1 job | $547 | $1,322 | $2,388 |
| Subtotal | $3,647 | $6,822 | $10,788 |
Notes
- • Estimate based on 300 sq ft bedroom addition with slab foundation and extended HVAC.
- • Prices vary significantly by region, contractor, and market conditions. Use this as a planning estimate and compare at least three written quotes.
- • Permits, engineering, and site conditions can significantly change the total cost.
About the Addition Cost Calculator
A home addition is a major investment that adds living space without the hassle of moving. Whether you need an extra bedroom, a family room, a sunroom, or a full in-law suite, our calculator breaks down the cost by construction phase — foundation, framing, roofing, exterior, MEP systems, and interior finishes — so you can see exactly where your budget goes.
How We Calculate Addition Cost
We calculate costs for each construction phase using typical ranges per square foot. Room type affects overall cost through a type multiplier: sunrooms (0.7x) are simpler builds, bedrooms (1.0x) are standard, family rooms (0.9x) need fewer fixtures, and in-law suites (1.3x) include kitchen and bathroom plumbing. HVAC costs depend on whether you extend the existing system or install a dedicated unit. Permits are estimated at 3-5% of the construction subtotal.
Factors That Affect Addition Cost
Key factors affecting home addition cost: addition size, room type (in-law suites with kitchen and bath cost the most), foundation type, whether you build out or up, HVAC approach, quality of interior finishes, how easily the addition ties into the existing structure, local labor rates, and permit requirements. Matching the existing roofline and exterior adds complexity and cost.
Planning a larger project? You may also want to estimate costs for cost to build a house, basement finishing cost, or foundation repair cost.
Sources
- • HomeAdvisor cost guides
- • Fixr.com cost guides
- • contractor survey benchmarks
Costs are based on current industry ranges and vary by location and market conditions.