Rex says
Money math without the spreadsheet headache. Plug in your numbers and I'll show you exactly where the dollars land.
Try a scenario
Click to load — tweak from there.Inputs
Applies a regional labor-and-bid multiplier to the whole job.
Result
Estimated addition cost
$154,905
Cost per square foot
$387
Shell and finishes
$104,000
Foundation
$15,200
Bathroom / kitchen premium
$0
Contingency reserve
$20,205
Low end of band
$136,316
High end of band
$185,886
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How to use this
- 1Enter addition footprint (sq ft).
- 2Enter shell and finish rate ($/sq ft).
- 3Enter region — Applies a regional labor-and-bid multiplier to the whole job..
- 4Enter foundation type.
- 5Enter structural tie-in to existing wall/roof ($).
- 6Enter hvac extension or mini-split ($).
- 7Enter bathrooms or kitchens inside the addition (rooms).
- 8Enter cost per wet room ($).
- 9Enter contingency (%).
- 10Read your estimated addition cost on the right — it updates as you type.
- 11Hit Share to keep the scenario or send it to someone.
About this calculator
An addition costs more per square foot than new construction, and the reason is structural rather than cosmetic: you pay for a new foundation and roof on a small footprint, you pay a crew to open and re-tie the existing wall, and you pay for a week of protecting a house someone is living in. Kitchens and bathrooms inside an addition carry their own premium because of the plumbing and cabinetry, which is why one number per square foot never fits every addition. This calculator prices the shell per square foot with a regional index, then adds foundation type, the wall tie-in, HVAC extension, and an optional wet-room premium. Worked assumption for the defaults: a 400 sq ft family-room addition at $260/sq ft on a crawl space, $9,000 tie-in, $6,500 HVAC extension, no plumbing, 15% contingency — about $155,000 in a national-average market and roughly $205,000 on the West Coast.
Worked example
Using the values the calculator loads with:
Inputs
- Addition footprint: 400 sq ft
- Shell and finish rate: 260 $/sq ft
- Region: US national average — 1.00×
- Foundation type: Crawl space — $38/sq ft
- Structural tie-in to existing wall/roof: 9000 $
- HVAC extension or mini-split: 6500 $
- Bathrooms or kitchens inside the addition: 0 rooms
- Cost per wet room: 24000 $
- Contingency: 15 %
Results
- Estimated addition cost: $154,905
- Cost per square foot: $387
- Shell and finishes: $104,000
- Foundation: $15,200
- Bathroom / kitchen premium: $0
- Contingency reserve: $20,205
- Low end of band: $136,316
- High end of band: $185,886
What each field means
Inputs
- Addition footprint (sq ft)
- The addition footprint used in the calculation, measured in sq ft. Starts at 400 sq ft so you have a working example on load. Accepted range: 40–3000 sq ft.
- Shell and finish rate ($/sq ft)
- The shell and finish rate used in the calculation, measured in $/sq ft. Starts at 260 $/sq ft so you have a working example on load. Accepted range: 120–800 $/sq ft.
- Region
- Applies a regional labor-and-bid multiplier to the whole job.
- Foundation type
- Pick the option that matches your situation — the maths changes per option. Choices: Slab on grade — $22/sq ft, Crawl space — $38/sq ft, Full basement — $62/sq ft.
- Structural tie-in to existing wall/roof ($)
- The structural tie-in to existing wall/roof used in the calculation, measured in $. Starts at 9000 $ so you have a working example on load. Accepted range: 0–80000 $.
- HVAC extension or mini-split ($)
- The hvac extension or mini-split used in the calculation, measured in $. Starts at 6500 $ so you have a working example on load. Accepted range: 0–40000 $.
- Bathrooms or kitchens inside the addition (rooms)
- The bathrooms or kitchens inside the addition used in the calculation, measured in rooms. Starts at 0 rooms so you have a working example on load. Accepted range: 0–4 rooms.
- Cost per wet room ($)
- The cost per wet room used in the calculation, measured in $. Starts at 24000 $ so you have a working example on load. Accepted range: 0–120000 $.
- Contingency (%)
- The contingency used in the calculation, measured in %. Starts at 15 % so you have a working example on load. Accepted range: 0–35 %.
Results
- Estimated addition cost
- Returned as a money amount in US dollars and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Cost per square foot
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Shell and finishes
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Foundation
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Bathroom / kitchen premium
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Contingency reserve
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Low end of band
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- High end of band
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
Where does the regional multiplier come from?
It is a job-level index against a US national average of 1.00, built from the spread contractors quote between low-cost rural markets and the highest-cost coastal metros. Materials vary less than labor, but bids move together, so one index tracks real quotes better than splitting them. Swap in your own multiplier by picking the tier closest to two bids you already have.
Why is an addition pricier per square foot than a new house?
Fixed costs land on a tiny footprint. A 400 sq ft addition needs its own foundation, roof, and crew mobilisation, and the crew works around a family living in the house. Doubling the size rarely doubles the price.
Is building up cheaper than building out?
Sometimes — you skip the foundation but pay for structural reinforcement, stairs, and often a full re-roof. Run the second-story tool and compare the two totals for your own house.
Why does the tool add a contingency?
Because renovation scopes change once walls are open. On finished-space work 10–15% is normal; on anything touching foundations, sewer lines, or 1970s-and-older wiring, 20% is the honest number. If your contingency is never spent, you finished under budget — that is the point.
Accuracy and limitations
- Results are estimates before tax, fees, and inflation unless an input explicitly covers them.
- Rates are treated as fixed for the whole period — variable-rate products will drift from this projection.
- This is educational maths, not financial advice. Check anything contractual with the lender or your accountant.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Cost to Build a Home Addition Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/cost-to-build-a-home-addition-calculator
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/cost-to-build-a-home-addition-calculator" target="_blank" rel="noopener">Cost to Build a Home Addition Calculator — RevenueLab</a> (2026).</p>
Source: [Cost to Build a Home Addition Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/cost-to-build-a-home-addition-calculator) (2026).