2026 Home Addition Cost by Type
| Addition Type | Cost/SF | 400 SF Addition | Timeline |
|---|---|---|---|
| Bump-Out (under 100 sq ft) | $200-$400 | $80,000.00 - $160,000.00 | 2-4 weeks |
| Single-Room Addition (ground level) | $150-$250 | $60,000.00 - $100,000.00 | 2-4 months |
| Primary Suite Addition | $200-$350 | $80,000.00 - $140,000.00 | 3-5 months |
| Second-Story Addition | $180-$350 | $72,000.00 - $140,000.00 | 4-7 months |
| Sunroom / Four-Season Room | $100-$200 | $40,000.00 - $80,000.00 | 3-6 weeks |
| Attached Garage (unconditioned) | $40-$70 | $16,000.00 - $28,000.00 | 3-6 weeks |
How Much Does It Cost to Build an Addition?
A home addition costs $150 to $350 per square foot in 2026 for conditioned living space, all trades included. A typical 400 sq ft single-room addition runs $75,000 to $131,000 once you add the foundation, permits, an HVAC zone, and a realistic contingency.
The per-square-foot number moves in both directions. A bump-out under 100 sq ft costs $200-$400/sf because you mobilize the same crew, pull the same permit, and tie into the same roof for a fraction of the floor area. An unconditioned attached garage drops to $40-$70/sf because there is no insulation, drywall, or finish flooring. Square footage alone never tells the story.
The line most homeowners skip is contingency. I have opened walls on a simple addition tie-in and found rot, knob-and-tube wiring, and a header that was never sized for the load above it. Budget 15% to 20% on top of the bid. If you do not spend it, you have a nicer floor.
What Drives Each Addition Type
Bump-Out (under 100 sq ft)
$200-$400/sf · 2-4 weeks
Highest cost per sq ft. You pay for the same crew mobilization, permits, and roof tie-in on a fraction of the floor area.
Single-Room Addition (ground level)
$150-$250/sf · 2-4 months
The baseline addition. New foundation, framing, roof tie-in, and a full set of trades.
Primary Suite Addition
$200-$350/sf · 3-5 months
Bedroom plus a full bath. Plumbing rough-in and tile work drive the premium over a plain bedroom.
Second-Story Addition
$180-$350/sf · 4-7 months
No new foundation, but you tear off the existing roof, verify the structure below, and the house is exposed to weather.
Sunroom / Four-Season Room
$100-$200/sf · 3-6 weeks
Cheapest per sq ft because it is mostly glass and framing. Add HVAC if you want it usable in January.
Attached Garage (unconditioned)
$40-$70/sf · 3-6 weeks
No insulation, drywall, or finish flooring. Slab, framing, roof, and a door.
Build Your Custom Estimate
Scope Add-Ons
Home Addition Estimate: Single-Room Addition (ground level)
Addition Size
400 sq ft
Total Estimated Cost
$75,095.00 - $130,755.00
| Category | Low Estimate | High Estimate |
|---|---|---|
| Shell & Trades (400 sf × $150-$250 × 1x) | $60,000.00 | $100,000.00 |
| Foundation: Concrete Slab ($5-$8/sf) | $2,000.00 | $3,200.00 |
| HVAC Extension or New Zone | $2,000.00 | $5,000.00 |
| Electrical Subpanel | $800.00 | $2,500.00 |
| Permits & Plan Review | $500.00 | $3,000.00 |
| Contingency (15% of subtotal) | $9,795.00 | $17,055.00 |
| Total | $75,095.00 | $130,755.00 |
Ways to Bring the Number Down
- Build up instead of out when the existing foundation and framing can carry the load. You skip $5-$25/sf of new foundation.
- Keep plumbing on a wall that backs up to existing supply and drain lines. Moving a stack across the house adds $3,000-$8,000.
- Stack the addition roof line with the existing one. Complex tie-ins and valleys add 10-20% to framing and roofing.
- Drop from high-end to mid-range finishes. That single change cuts roughly 26% off the shell and trades line.
- Get three bids on the same written scope. The cheapest bid is usually the one missing line items.
12,800+ estimates calculated this month
Last updated: July 8, 2026. Cost ranges reflect 2026 national averages and are anchored to the EstimationPro pricing reference. Prices vary by region, so get multiple local bids before you commit.
Where the Money Goes on a 400 SF Addition
A mid-range 400 sq ft room addition on a slab, with permits, an HVAC zone, and a subpanel, prices out around $75,100 on the low end and $130,800 on the high end once you carry a 15% contingency. Here is how that splits.
| Line Item | Low | High | Basis |
|---|---|---|---|
| Shell & trades | $60,000 | $100,000 | 400 sf × $150-$250/sf |
| Slab foundation | $2,000 | $3,200 | 400 sf × $5-$8/sf |
| Permits & plan review | $500 | $3,000 | Residential building permit |
| HVAC extension or zone | $2,000 | $5,000 | One new conditioned zone |
| Electrical subpanel | $800 | $2,500 | Subpanel install |
| Contingency | $9,795 | $17,055 | 15% of subtotal |
| Total | $75,095 | $130,755 | $188-$327 per sq ft |
Pricing a specific room instead of the whole envelope? The room addition cost calculator breaks the same job down by trade percentage, and the second story addition calculator handles the structural side of building up. For an unconditioned bay, use the garage cost calculator.
Home Addition Cost Planning Guide
Cost drivers, contingency, and the line items homeowners forget on addition projects.
What Drives Home Addition Cost the Most?
Addition type and finish level move the number more than square footage does. A 400 sq ft bump-out and a 400 sq ft garage cost the same crew the same weeks, but land $60,000 apart because one is conditioned living space and one is a slab with a door.
- Addition type: $40-$70/sf for an unconditioned garage vs. $200-$400/sf for a bump-out under 100 sq ft
- Finish level: builder-grade runs 0.85x, high-end runs 1.35x. That spread is about 60% on the same shell
- Foundation: slab $5-$8/sf, crawlspace $7-$12/sf, full basement $15-$25/sf. Second stories skip this entirely
- Plumbing distance: a bath on a wall backing existing supply and drain lines saves $3,000-$8,000 over a new stack run
- Roof tie-in: valleys and mismatched pitches add 10-20% to framing and roofing
Key Takeaways
- Type + finish drive more cost than raw square footage
- Foundation adds $5-$25/sf; second stories add $0
- Finish level alone swings the shell cost about 60%
Why Small Additions Cost More Per Square Foot
A 60 sq ft bump-out can cost $200-$400 per square foot while a 600 sq ft addition runs $150-$250. The fixed costs do not shrink with the floor plan.
- Permits: $500-$3,000 whether the addition is 60 sq ft or 600 sq ft
- Mobilization: every sub shows up, sets up, and bills a minimum regardless of size
- Roof tie-in: cutting into the existing roof and flashing it costs the same on a small bump-out
- Design and engineering: $2,000-$8,000 for a stamped set, and a small addition still needs one if it touches structure
Going from 100 sq ft to 200 sq ft adds roughly $15,000-$25,000 in shell and trades, and almost nothing in permits, design, or mobilization. That second 100 sq ft is the cheapest square footage you will ever buy. Size up while the crew is already there.
Key Takeaways
- Permits, mobilization, and roof tie-in do not scale down
- Bump-outs: $200-$400/sf vs. $150-$250/sf for a full room
- Marginal square footage is far cheaper than the first 100 sq ft
The Contingency Line Nobody Budgets
Budget 15% to 20% on top of the accepted bid. Additions tie into an existing house, and the existing house is where the surprises live.
- Rot at the tie-in: common in wet climates, especially where the old roof meets a wall
- Undersized headers and joists: a second story means the framing below has to carry the new load, and older homes often need sistering at $100-$325 per joist
- Code upgrades: touch the panel and you may owe a 200-amp service at $1,500-$4,000
- Knob-and-tube or failing supply lines: found on demo day, not on bid day
An experienced contractor prices contingency into the bid. A cheap bid usually means the hidden scope was left out on purpose, and you will meet it later as a change order.
Key Takeaways
- Add 15-20% over the bid for hidden conditions
- Sistering joists: $100-$325 each; panel upgrade: $1,500-$4,000
- The cheapest bid is usually the one missing line items
How to Use This Calculator
Find your addition type in the cost table
Start with the cost-by-type table. A bump-out, a ground-level room addition, a second story, and an unconditioned garage all price differently per square foot, and the spread runs from $40/sf to $400/sf.
Set the size and finish level
Pick a preset size or enter custom square footage, then choose builder-grade, mid-range, or high-end. Finish level alone swings the shell and trades cost by roughly 60% from bottom to top.
Add the foundation and scope add-ons
Select slab, crawlspace, or full basement. Second stories skip this line entirely. Then toggle the bathroom, kitchenette, HVAC zone, subpanel, permits, and design fees that apply to your scope.
Leave the contingency on
The 15% contingency covers what you find after demo: rot at the tie-in, undersized joists, code upgrades. Review the itemized low-to-high breakdown, then print or share the estimate.
Home Addition Cost Math
Total = (Shell + Foundation + Add-Ons) x (1 + Contingency)
Shell = Square Feet x Type Rate x Finish Multiplier
Foundation = Square Feet x Foundation RateWhere:
- Bump-Out
- = $200-$400 per sq ft, under 100 sq ft
- Room Addition
- = $150-$250 per sq ft, ground level
- Primary Suite
- = $200-$350 per sq ft, includes full bath
- Second Story
- = $180-$350 per sq ft, no new foundation
- Sunroom
- = $100-$200 per sq ft, four-season
- Attached Garage
- = $40-$70 per sq ft, unconditioned
- Finish Multiplier
- = 0.85x builder, 1.0x mid-range, 1.35x high-end
- Foundation Rate
- = $5-$8 slab, $7-$12 crawlspace, $15-$25 basement
- Contingency
- = 15% of subtotal for hidden conditions
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Frequently Asked Questions
How much does it cost to build an addition in 2026?
Is it cheaper to build up or build out?
Why do small additions cost so much per square foot?
How do contractors price a home addition for a client?
How much contingency should a homeowner budget for an addition?
Do you need a permit for a home addition?
How long does a home addition take to build?
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