How the estimate is built
Every space you enter as a new addition gets two allowances: $175 per sq ft for the shell (the new structure) and for tying it into your house, and a fit-out allowance for the room itself (the interior work). A space marked "Remodel existing space" is work inside your current house and gets the fit-out only. Both include labor.
The fit-out allowances are the same ones Project Pilot's estimator uses:
| Room | Base | Per sq ft |
|---|---|---|
| Kitchen | $11,200 | $210 |
| Bathroom | $5,600 | $168 |
| Basement | $8,400 | $49 |
| Bedroom | $2,100 | $49 |
| Closet | $1,400 | $70 |
| Laundry / washer-dryer relocation | $4,200 | $70 |
| Living space / office | $2,100 | $49 |
These are fixed planning numbers, not live prices from your area.
The reserve is a range because how much goes wrong varies. HUD's 203(k) renovation loan program requires a contingency reserve of 10% to 20% on homes 30 years or older. If your house is older, plan nearer the top of that range.
The default bedroom and bath addition, line by line
The bedroom's shell is $52,500 and its fit-out $16,800. The bathroom's are $14,000 and $19,040. Per square foot, the bathroom costs $413 and the bedroom $231. So a single per-square-foot rate for a whole addition is really a guess about what you'll put inside it.
Four 2025 addition projects, priced nationally
The Cost vs. Value 2025 report prices four addition projects as national average job costs, from $60,645 for a 48 sq ft midrange bath to $111,255 for a 100 sq ft upscale bath and $351,613 for a 640 sq ft upscale suite, each with its scope written out by the Journal of Light Construction.
Job cost rises with scope, not just size
Each one is a complete job, and the scope is where the money is. The midrange suite, at $170,517, is a 24 by 16 ft bedroom with a walk-in closet, a soaker tub, a separate shower and a double vanity. If the closest project costs far more than your total, compare its scope to yours before you set a budget.
What to ask about before bids
Budgets slip when the scope grows. In the Houzz & Home Study 2026, 37% of renovating homeowners went over their budget in 2025, across all project types. Houzz found 35% chose higher-end materials than planned and 31% expanded the scope mid-project.
Before you compare contractor prices, ask each one whether their number covers:
- Surprises at the foundation or at the wall where the addition joins your house
- Engineering and design drawings
- Permits and inspection fees
- The finish choices you've made so far, and allowances for the ones you haven't
Start with the free readiness assessment. It turns your addition into a list of open decisions and scope gaps to settle before you ask for bids. When the bids come in, Bid Room (part of Project Pilot Complete, $199 once per project) lines them up against one normalized scope, so each contractor's allowances and missing scope sit next to their total.
Questions people ask
How much would a 20x20 addition cost?
A 20x20 addition is 400 sq ft. As one new bedroom or living space, the calculator puts it at a preliminary $91,700, or $100,870 to $110,040 with the reserve. Make 80 sq ft of it a bathroom and it's $106,820 before the reserve.
How much does it cost to add 1,000 square feet?
As 1,000 sq ft of new living space, the calculator gives a preliminary $226,100, or $248,710 to $271,320 with the reserve. If yours mixes rooms, enter each one and keep the areas adding up to 1,000 sq ft.