A 250 sq ft room addition on a crawlspace, built to a mid-range finish, costs about $53,000 at US-average prices, around $212 per sq ft all in, with a range of $38,500 to $65,500. That matches HomeAdvisor's average home addition of about $51,000 and Fixr's $50,000 for a 250 sq ft bedroom. A bathroom addition costs far more per square foot, and a second story far more in total.
How the home addition cost is calculated
Additions are priced by the square foot, built and finished, because that is how the published guides report them. The price per square foot depends on what you add:
| Type of addition | Price per sq ft, built and finished | Sources |
|---|---|---|
| Bump-out | $90 to $300 | HomeAdvisor |
| Room addition | $80 to $250 | HomeAdvisor ($80 to $210), Fixr bedroom ($150 to $250) |
| Bathroom addition | $400 to $1,263 | Fixr ($400 to $550), Cost vs Value 2025 ($1,113 to $1,263) |
| Second story | $100 to $500 | HomeAdvisor ($100 to $300, up to $500 premium) |
| Sunroom, four-season | $130 to $300 | Fixr (2022) |
Total = added area (sq ft) × price per sq ft for the type and level + basement premium + site prep + permits + 10% contingency
The per-foot prices already include a slab or crawlspace. A full basement under the addition adds the difference Fixr gives between a basement ($20 to $50 per sq ft) and a crawlspace ($10 to $18): $10 to $32 per sq ft. Ground-level additions also carry excavation and site prep at $1,500 to $5,800 (HomeAdvisor). Materials are nearly two-thirds of the cost according to HomeAdvisor's second-story guide, so labor is taken as 35% of shell prices and 50% for bathrooms (Fixr), then scaled by your state's construction wages (BLS) while materials follow its goods prices (BEA).
Worked example: a 250 sq ft room
| Line | Typical |
|---|---|
| Room addition, built and finished: 250 sq ft × $172.50 | $43,000 |
| Excavation and site prep | $3,650 |
| Permits | $1,600 |
| Contingency, 10% | $4,700 |
| Total | $53,000 (range $38,500 to $65,500) |
With a full basement instead of a crawlspace the same room costs about $59,000. Size the footings and concrete with the concrete footing calculator, the walls with the framing calculator, and the new roof with the roofing calculator.
Cost by type of addition
| Addition, mid-range | Size | Typical |
|---|---|---|
| Bump-out | 40 sq ft (4 × 10 ft) | $14,200 |
| Room addition | 250 sq ft | $53,000 |
| Bathroom addition | 48 sq ft (6 × 8 ft) | $49,500 |
| Sunroom | 200 sq ft | $53,000 |
| Second story | 800 sq ft | $177,500 |
Bump-outs extend a room by a few feet without a full foundation; HomeAdvisor puts most between $10,000 and $43,000, averaging $27,000. Small ones are expensive per foot because site prep and permits do not shrink with the room.
Bathroom additions spread plumbing, tile and fixtures over very little floor, so the per-foot price is the highest here. Fixr's average bathroom addition is $20,000, but that includes converting space inside the house; its price for a bathroom in a new addition is $400 to $550 per sq ft. Cost vs Value 2025 prices a 6 × 8 ft midrange bathroom addition at $60,645 and a 100 sq ft upscale one at $111,255. The calculator's typical $832 per sq ft sits between them. To redo an existing bathroom instead, use the bathroom remodel cost calculator.
Second stories skip the foundation and site work but may need the existing foundation reinforced, which HomeAdvisor says starts at $5,000. Its average second-story project is $175,000, and 800 sq ft costs $80,000 to $240,000. Permits run $1,200 to $2,000.
Sunrooms built as true four-season rooms with a foundation and roof cost $130 to $300 per sq ft according to Fixr; prefabricated kits are $50 to $75 per sq ft. Fixr's sunroom figures date from 2022, the oldest source here, so treat them as a floor.
What drives the price
- Plumbing. Kitchens and bathrooms cost several times what a bedroom does per square foot.
- Foundation. Crawlspace and slab are in the base price; a basement adds $10 to $32 per sq ft.
- Tie-in to the house. Matching the roofline, siding and windows of the existing house adds labor that a stand-alone room would not have.
- Design. HomeAdvisor puts architect fees at 5% to 20% of the project; they are not in this estimate.
How to cut the cost
Build a simple rectangle under a simple roof, keep plumbing out of it unless it is a bathroom, and finish the interior yourself: drywall, paint and flooring are where DIY labor savings are real. If the space you need already exists as an unfinished basement, the basement finishing cost calculator will usually come out far cheaper, and reworking rooms you already have is priced by the home remodel cost calculator. All remodeling estimates are under project costs.
These are dated estimates from published national data adjusted for your state, not quotes. Additions need drawings, a permit and inspections; get bids on the same drawings.
A finished sunroom or three-season room is priced separately by the sunroom cost calculator.