Loan Home Addition Cost Estimator
Addition quotes confuse because a bump-out and a second story share nothing but the word "addition". This estimator separates the five types — room, bump-out, second story, garage conversion, sunroom — each with its own per-foot band.
The default 400 sq ft is a primary-suite or family-room scale addition. Kitchens and bathrooms inside an addition carry their own premium (plumbing, cabinetry, tile) — budget them as separate projects on top of the shell figure here.
How it works
Installed per-square-foot bands by type: room addition $200–$325 (foundation, framing, roof tie-in, systems), bump-out $250–$400 (small footprints concentrate structure and roof work), second story $225–$375 (no foundation, but structural reinforcement and a roof that comes off first), garage conversion $100–$175 (the shell exists — insulation, floors, HVAC and finishes are the job), sunroom $150–$275 (kit systems at the low end, site-built at the high). Design, permits and engineering typically add 8–15% on top — inside these bands' mid-range on simple jobs, extra on complex ones.
Method & worked example
Method: Total = addition sq ft × type band: room addition $200–325, bump-out $250–400, second story $225–375, garage conversion $100–175, sunroom $150–275 per sq ft, finished at standard specification.
Worked example: A 400 sq ft room addition: 400 × $200–325 = $80,000–$130,000, mid-band $105,000.
When to use this
- You are weighing an addition against moving house.
- A design-build firm quoted a per-foot number you want checked by type.
- You are comparing building out (room) vs up (second story) vs converting the garage.
How to use it, step by step
- Enter the addition's finished square footage.
- Pick the type that matches the structure — the band follows the type.
- Read the range, then budget kitchens/baths inside it separately.
- Add design and permits (8–15%) when moving from estimate to budget.
What the inputs mean
| Input | What it is | Typical values |
|---|---|---|
| Addition size | Finished square feet added | 200 sq ft bump-out; 400 sq ft suite; 800+ major |
| Type | Structure type sets the band | Garage conversion $100–175; room $200–325; bump-out $250–400 per sq ft |
Another worked example
A 200 sq ft kitchen bump-out: 200 × $250–400 = $50,000–$80,000, mid-band $65,000 — before the kitchen fit-out itself. Small additions carry the highest per-foot bands in residential work because every trade still has to show up for a tiny footprint.
Frequently asked questions
How much does a 400 sq ft room addition cost?
$80,000–$130,000 finished, mid-band about $105,000. That is $200–$325 per sq ft — more per foot than new construction, because tie-ins, roof matching and systems extension are concentrated work.
How much does a 200 sq ft bump-out cost?
$50,000–$80,000. Bump-outs shock people — the per-foot band ($250–$400) is the highest of the addition types because a kitchen bump-out still needs structure, roof, and every trade for a tiny footprint.
Is it cheaper to build out or up?
Per square foot, similar: ground-level room additions run $200–$325; second stories $225–$375 but may need stair space and structural work below. Building up wins when the lot cannot grow; building out wins when the foundation and roof are simple.
Why is a garage conversion so much cheaper?
The expensive bones exist: shell, roof, slab. The $100–$175 band buys insulation, raised flooring, HVAC extension, windows and finishes. Check local rules — many towns require the parking count to survive the conversion.
Do additions pay back at resale?
Family-room and primary-suite additions historically return roughly half to two-thirds of cost at resale in national surveys; garage conversions vary by market parking pressure. The honest framing: buy the space for living in, count resale as a rebate, not the reason.
🏘️ Pricing the whole house instead?
For new construction rather than an addition, the House Building Cost Estimator prices by tier and region — starter to luxury — with the two-story factor handled.
How long does an addition take?
Design and permits 2–4 months; construction 2–4 months for a room addition, longer for second stories. Weather and inspection queues stretch the calendar more than the work does.
Does an addition need its own HVAC?
Sometimes a zone or a mini-split; sometimes the existing system has capacity. The BTU Estimator sizes the added load, and the HVAC Estimator prices the equipment answer either way.