Home Addition Cost Calculator

By Uzair Arshad , Senior Civil and Structural Engineer

Last updated: April 20, 2026

This free home addition cost calculator helps you estimate home addition costs online in seconds. Use this room addition cost estimator and home extension cost calculator to compare sizes, materials, and finishes, and calculate accurate cost ranges based on real-world pricing.

Typical cost: $125 to $250 per square foot

1Type/
2Size/
3Finish/
4Features/
5Results

What kind of addition are you planning?

Choose the closest match. You can refine your estimate in the next steps. Cost ranges shown are 2026 US averages per square foot.

How to use this calculator

The home addition cost calculator runs in five short steps. Each step takes about ten seconds, and the result shows a low-to-high cost range, home addition cost per square foot, a trade-by-trade breakdown, and a typical timeline.

Whether you are pricing a room addition, home extension, or second floor addition, this cost estimator gives you a realistic starting number. Use it as a DIY home addition cost calculator for early budgeting or to sanity-check contractor quotes before signing.

Cost per square foot

Most standard projects land between $125 and $250 per sq ft in 2026. Final pricing changes with room type, finish level, structural scope, permit fees, and whether you need new plumbing or HVAC. Use this range as your base, then adjust with project-specific upgrades.

Example costs by common room size

Room Size	Estimated Cost
10x10 (100 sq ft)	$12,500 to $25,000
10x20 (200 sq ft)	$25,000 to $50,000
12x24 (288 sq ft)	$36,000 to $72,000
20x20 (400 sq ft)	$50,000 to $100,000

How to calculate cost

To calculate home addition cost, use a simple formula: total cost = square footage x cost per sq ft. This free home addition cost calculator uses that base math, then adjusts for finish level and optional features.

Example: a 12x24 addition is 288 square feet. At $125 to $250 per sq ft, the base range is $36,000 to $72,000 before permit fees and specialty upgrades. A two story addition on the same footprint doubles the livable area but also adds structural reinforcement costs.

Need more context? See our cost per square foot guide, try the renovation cost calculator, or compare tools in our construction cost estimator hub.

Pick the addition type closest to your project. Each type carries a different per-square-foot range because the trades involved differ.
Enter the addition size as length and width in feet, or skip ahead and enter total square footage directly.
Pick a finish level. Standard covers builder-grade fixtures, mid-range fits most family budgets, high-end covers custom cabinetry and stone counters.
Check optional features that apply. Most additions need foundation work, permits, and HVAC at minimum.
Click "Calculate home addition cost" to see your range, breakdown, and timeline.

Pro tip: get three contractor bids from licensed local builders and use this estimate as a sanity check. Bids more than 30% below the range usually mean the contractor missed something on the scope. Bids more than 50% above usually mean expensive finishes or unexpected structural work.

2026 cost ranges by addition type

These are the per-square-foot ranges the wizard uses, sourced from US national contractor surveys updated for 2026.

Addition Type	Cost per Sq Ft	Typical 400 Sq Ft Total	Timeline
Bedroom	$80 to $200	$32,000 to $80,000	8 to 12 weeks
Bathroom	$250 to $800	$100,000 to $320,000	10 to 16 weeks
Kitchen extension	$250 to $750	$100,000 to $300,000	10 to 18 weeks
Master suite	$150 to $350	$60,000 to $140,000	10 to 16 weeks
Sunroom	$150 to $400	$60,000 to $160,000	6 to 10 weeks
Second story	$100 to $500	$40,000 to $200,000	12 to 20 weeks
Garage	$30 to $100	$12,000 to $40,000	4 to 8 weeks
In-law suite	$100 to $300	$40,000 to $120,000	10 to 16 weeks
Bonus or media room	$100 to $175	$40,000 to $70,000	6 to 12 weeks

How the calculation works

Base Cost:
Base cost = Square footage × Cost per sq ft (for your room type)
Cost per sq ft = Adjusted by finish level (lower, middle, or upper third of the type range)

Feature Costs:
Feature costs = Sum of selected upgrades (per-sqft features × area, or flat dollar amounts)
Total cost = Base cost + Feature costs (shown as a low to high range)

Trade Breakdown:
Trade breakdown = Total cost × Share % for each of 8 trade categories

Square footage: Total floor area of the addition in square feet
Cost per sq ft: 2026 price range for the chosen room type, adjusted by finish level
Finish level: Standard, mid, or high-end. Selects the lower, middle, or upper third of the cost range.
Feature costs: Optional upgrades like foundation, plumbing, HVAC, permits, and roofline work

The home addition cost calculator multiplies square footage by cost per square foot for your room type. The cost per square foot comes from a range that depends on the room and the finish level you pick.

We split each type's range into thirds. Standard finish uses the lower third, mid-range finish uses the middle third, and high-end finish uses the upper third. A bedroom at standard finish runs $80 to $120 per square foot. The same bedroom at high-end finish runs $160 to $200 per square foot. The mid-range tier covers the middle.

Example calculation: 300 sq ft master suite

Here is how the home addition cost calculator handles a real project:

Given: 300 sq ft master suite, mid-range finish, with foundation and HVAC

Step 1: Master suite range is $150 to $350 per sq ft. Mid-range finish uses the middle third: $217 to $283 per sq ft.

Step 2: Base cost = 300 × $217 to 300 × $283 = $65,100 to $84,900

Step 3: Foundation (per-sqft, mid-range): 300 × $20 to 300 × $25 = $6,000 to $7,500

Step 4: HVAC (flat): $3,000 to $7,000

Step 5: Total = $65,100 + $6,000 + $3,000 to $84,900 + $7,500 + $7,000 = $74,100 to $99,400

This home addition construction cost estimate gives you a planning range. Your actual bids will land somewhere in this spread depending on your local market, contractor markup, and material choices.

For a deeper look at how room type, finish level, and complexity drivers shift pricing, read our home addition cost per square foot guide.

Optional features add to the base. Foundation work and plumbing scale with square footage because both trades price by area. Electrical panel upgrades, HVAC extensions, roofline tie-ins, and permits add a flat dollar range because these trades price by job, not by area.

Why the home addition cost calculator returns a range

Material prices vary by region, contractor markup runs 10% to 25%, and skilled labor in major metros costs 30% to 50% more than rural areas. The home addition cost calculator uses a range to cover the realistic spread of bids you should expect to receive on the same set of plans.

Treat the low end as your floor and the high end as your ceiling. Most projects land in the middle 40% of the range. If every bid you get sits below the floor, double-check the contractor's license, insurance, and references before signing.

Home addition cost breakdown by trade

The breakdown table in your result shows what share of the total budget each trade typically claims. Finishing wins at roughly a quarter of the total because it covers drywall, paint, flooring, trim, doors, and fixtures combined. Foundation runs 12% to 14% on slab-on-grade work. Framing follows at 16% to 18%, then plumbing, electrical, and HVAC each take 8% to 15%.

Permits and inspection fees stay small at 4% but vary widely by jurisdiction. A simple addition in a rural county may cost $400 in fees. The same addition in a coastal California city often runs $5,000 in permit and impact fees. Check your local building department before you set the budget.

Contractor overhead and profit covers 13% to 15%. This pays for the contractor's office, insurance, vehicle costs, and project management time. A bid with no overhead line item is hiding it inside the trade prices, not skipping it.

2026 cost data sources

Cost ranges come from US national contractor surveys updated for 2026. Lumber prices stabilized in late 2025 after three years of swings. Drywall and concrete are up 8% to 12% from 2024. Skilled labor (framers, electricians, plumbers) is up 6% to 10% year over year because the trades shortage continues. The home addition cost calculator bakes these factors into the per-square-foot ranges.

Assumptions and limitations

This calculator assumes US national average pricing for 2026. Your actual costs may differ based on local labor rates, material availability, and site conditions. The estimate does not include landscaping, furniture, appliances, or interior decorating.

Second floor addition costs assume the existing foundation can support the load. If your foundation needs reinforcement, add $10,000 to $30,000 to the estimate. A structural engineer's report ($1,500 to $3,000) confirms whether your foundation qualifies.

This calculator is a planning tool, not a contract quote. Always get at least two to three written bids from licensed contractors in your area before setting your final budget. Actual construction cost varies by season, contractor availability, and local building code requirements.

Tips to save on your home addition

Bid the same scope to three contractors. Same drawings, same finish schedule, same timeline. Different scopes make bids impossible to compare apples-to-apples.

Pick a stock plan instead of a custom design. A stock addition plan runs $300 to $1,200 from a plan service. A custom architect for the same footprint runs $3,000 to $15,000 plus revision rounds.

Build over an existing foundation when you can. A bump-out over an existing slab or basement saves 12% to 14% of the total cost because you skip the most expensive line item.

Schedule the work for late winter or early spring. Contractor calendars are softest from January through March in most US markets, and competitive bids drop 5% to 10% during the slow season.

Pick fixtures yourself from a supply house. Markup on contractor-supplied fixtures runs 20% to 40%. Buying your own toilet, faucets, and lights at a plumbing supply or big box store keeps that markup in your pocket. Confirm the spec matches what your plumber will install before you buy.

Common home addition mistakes

Skipping the structural engineer on second story additions. Existing walls were not designed to carry a second floor. A $2,000 engineer's report can save you from $30,000 in retrofit framing later.

Forgetting setback rules. Many lots have 5 to 10 foot side setbacks and 25 foot rear setbacks. Build into a setback by accident and you may have to tear the work down. Pull a copy of your plot plan from the assessor before you draw.

Underestimating HVAC. A 400 square foot addition usually needs more than an extra duct run. The existing furnace or AC unit is often at capacity already, and a sub-zoning system or mini-split costs less than upsizing the whole house.

Frequently Asked Questions

How much does a home addition cost?

Most additions cost $125 to $250 cost per square foot in 2026, so a 200 square foot project often lands near $25,000 to $50,000. Plumbing-heavy or custom layouts can rise past $300 per square foot. Use this as a planning range, then validate it with contractor quotes.

How do you calculate home addition costs?

To calculate home addition cost, multiply square footage by your target rate, then add fixed costs. Example: 300 square feet x $175 equals $52,500, then add $5,000 for permits and HVAC for about $57,500. This gives you a practical early estimate before final bids arrive.

How much does it cost to add a room?

Adding one room often costs $125 to $250 cost per square foot for standard work. A 10x12 room, 120 square feet, usually runs $15,000 to $30,000. If you add plumbing, structural framing, or custom cabinets, the same footprint can rise to $35,000 to $50,000 in many markets.

What is the average cost per square foot for a home addition?

Home addition pricing usually falls between $125 and $250 per square foot for standard projects in 2026. Bedrooms may run $80 to $200, while bathrooms and kitchens often run $250 to $800. Permit fees, finish level, and mechanical upgrades create most of the pricing spread.

How much does a 20x20 addition cost?

A 20x20 addition is 400 square feet, so many projects land around $50,000 to $100,000 at common market rates. To calculate home addition cost for this size, start with your square footage rate, then add permits and trade upgrades. Bathroom or kitchen-heavy layouts can push totals beyond $120,000.

How much does a second story addition cost?

A second story addition costs $100 to $500 per square foot in 2026, depending on size, structural work, and finish level. A typical 400 square foot second floor addition runs $40,000 to $200,000. Existing foundations often need reinforcement, which adds $10,000 to $30,000 to the total project budget.