How much does a home addition cost per square foot?

A home addition typically costs $110-$300 per square foot depending on room type and finishes. A 3-season sunroom runs $110-$190/sqft, a bedroom $130-$200/sqft, and an in-law suite with kitchen and bathroom $180-$300/sqft. Foundation type, HVAC needs, and local labor rates all affect the final number.

Is it cheaper to build out or build up?

Building up (adding a second story) can be cheaper if the existing foundation and structure can support the additional load, since you avoid foundation and roofing costs for the added space. However, it requires structural engineering analysis and the home must be vacated during construction. Building out is more straightforward but requires new foundation and roof work.

Do I need a permit for a home addition?

Yes, virtually all home additions require a building permit. Most jurisdictions also require separate permits for electrical, plumbing, and mechanical work. Permit costs typically run 3-5% of construction cost. Building without permits can result in fines, forced removal, and problems when selling your home.

How long does a home addition take?

A small addition (100-200 sq ft) typically takes 2-3 months. A medium addition (300-500 sq ft) takes 3-5 months. A large in-law suite or second-story addition can take 5-8 months. Weather, permit delays, and material availability can extend timelines.

Is a home addition worth the investment?

A well-planned addition often makes sense financially. Bedroom additions typically recoup 50-60% of cost at resale, while family rooms recoup 60-70%. An in-law suite can add significant value if the local market demands multi-generational living. Compare the addition cost to the cost of buying a larger home — in many markets, adding on is significantly cheaper than moving.

Is a 'house extension' the same as a home addition?

Yes — "house extension," "home add-on," and "bump-out" all describe the same project: adding new square footage to an existing house. "Extension" is the common term in the UK and Australia, while "addition" and "add-on" are standard in the US. A small bump-out (pushing one room out a few feet) is the cheapest form; a full extension with its own foundation and roof costs the per-square-foot rates shown above. Whatever your contractor calls it, the cost drivers are identical: size, room type, foundation, HVAC, and how cleanly it ties into the existing structure.

Home Addition Cost Calculator

Estimate the cost of a home addition based on size, room type, foundation, and HVAC — materials, labor, and total cost breakdown.

Project Details

Addition Size(sq ft)

Typical bedroom addition 150-300 sq ft, in-law suite 500-1000 sq ft

Room Type

Stories

Foundation Type

HVAC

Estimated Cost

Location

Low

$23,978

Average

$44,701

High

$65,700

Cost Breakdown

Materials

Item	Qty	Low	Mid	High
Concrete Slab Foundation	300 sq ft	$2,400	$4,500	$6,000
Framing (walls, ceiling, structure)	300 sq ft	$4,500	$6,900	$9,300
Roofing (match existing)	300 sq ft	$1,500	$3,000	$4,500
Exterior Siding (match existing)	624 sq ft	$3,120	$6,240	$9,360
Interior Finishes (drywall, paint, trim, flooring)	300 sq ft	$5,700	$10,500	$15,000
Subtotal		$17,220	$31,140	$44,160

Labor & Fees

Item	Qty	Low	Mid	High
Electrical (wiring, outlets, fixtures)	300 sq ft	$900	$1,500	$2,400
Plumbing (hose bib / minimal hookup)	1 job	$200	$500	$1,000
HVAC — Extend Existing System	1 job	$2,000	$3,500	$5,000
Permits, Engineering & Fees	1 job	$610	$1,466	$2,628
General Contractor Overhead & Profit	1 job	$3,048	$6,595	$10,512
Subtotal		$6,758	$13,561	$21,540

Notes

• Estimate based on 300 sq ft bedroom addition with slab foundation and extended HVAC.
• Prices vary significantly by region, contractor, and market conditions. Use this as a planning estimate and compare at least three written quotes.
• Permits, engineering, and site conditions can significantly change the total cost.

About the Addition Cost Calculator

A home addition — also called a house extension or a home add-on — is a major investment that adds living space without the hassle of moving. Whether you need an extra bedroom, a family room, a sunroom, or a full in-law suite, our calculator breaks down the cost by construction phase — foundation, framing, roofing, exterior, MEP systems, and interior finishes — so you can see exactly where your budget goes.

How We Calculate Addition Cost

We calculate costs for each construction phase using typical ranges per square foot. Room type affects overall cost through a type multiplier: sunrooms (0.7x) are simpler builds, bedrooms (1.0x) are standard, family rooms (0.9x) need fewer fixtures, and in-law suites (1.3x) include kitchen and bathroom plumbing. HVAC costs depend on whether you extend the existing system or install a dedicated unit. Permits are estimated at 3-5% of the construction subtotal.

Factors That Affect Addition Cost

Key factors affecting home addition cost: addition size, room type (in-law suites with kitchen and bath cost the most), foundation type, whether you build out or up, HVAC approach, quality of interior finishes, how easily the addition ties into the existing structure, local labor rates, and permit requirements. Matching the existing roofline and exterior adds complexity and cost.

Home Addition Cost by Common Dimensions (2026)

If you're sizing an addition by its footprint — a 12×24, a 20×20 — this is the quickest way to set a budget. Figures are this calculator's low / average / high total for a 1-story bedroom addition on a slab foundation with extended HVAC, computed directly from the model below and rounded to the nearest $100. The Low column reflects budget finishes and lower-cost regions; High reflects premium finishes and high-cost metros; Average is the mid case. Other room types shift the total predictably: a sunroom runs roughly 0.85× these figures, a family room lands within a few percent of them, and an in-law suite with its own kitchen and bath about 1.25–1.3×.

Dimensions	Floor Area	Low	Average	High
10 × 12	120 sq ft	$12,000	$22,600	$33,500
10 × 20	200 sq ft	$17,400	$32,600	$48,000
12 × 16	192 sq ft	$16,900	$31,600	$46,600
12 × 20	240 sq ft	$20,000	$37,500	$55,200
12 × 24	288 sq ft	$23,200	$43,300	$63,600
16 × 20	320 sq ft	$25,300	$47,100	$69,200
20 × 20	400 sq ft	$30,400	$56,600	$83,000

Source: Computed directly from this calculator (bedroom, 1-story, slab foundation, extend HVAC; see costBasis), rounded to the nearest $100. Low/average/high span budget-to-premium finishes and market. Enter your exact size and room type above for a tailored range.

Cost Per Square Foot by Addition Type (2026)

Same square footage, very different total cost. At 300 sq ft, an in-law suite costs about 45% more than a sunroom because of the added plumbing, HVAC, and kitchen/bath finish work. Totals below are this calculator's mid estimate for a 300 sq ft addition; use the per-sq-ft range to set a realistic budget before getting quotes.

Addition Type	Cost Range ($/sq ft)	Mid Total (300 sq ft)	What Drives Cost
Sunroom (3-season)	$110 – $190	$39,000	Lighter build, no plumbing; HVAC optional
Garage conversion	$80 – $200	$28,000	Footprint exists; cost is finishing + insulation
Bedroom (no plumbing)	$130 – $200	$44,700	Full foundation + roof + HVAC extension
Family room (open span)	$140 – $210	$43,500	Larger windows, simple layout
In-law suite (kitchen + bath)	$180 – $300	$56,800	Full kitchen + bath plumbing/electrical
Second-story (build up)	$150 – $350	$70,000	Foundation upgrade + temporary roof removal
Full bathroom addition	$200 – $400	$45,000	Plumbing supply + drain + venting

Source: Calc-modeled types (sunroom/bedroom/family/in-law) from this calculator's 2026-calibrated model (see costBasis); garage/second-story/bath rows from HomeGuide & Angi 2026 cost guides.

Resale ROI by Addition Type (2024 Cost vs. Value Report)

Remodeling Magazine's annual Cost vs. Value Report tracks what each renovation type adds to a home's sale price. Two things stand out: (1) you almost never recoup 100% of an addition's cost, and (2) lower-spec additions outperform luxury ones on ROI. The right question isn't "will I recoup my money" — it's "is the recouped portion plus the years of use worth it?"

Addition Type	Avg Project Cost	Avg Resale Value Added	ROI
Bathroom addition (midrange)	$58,586	$24,656	42.1%
Master suite addition (midrange)	$167,931	$60,727	36.2%
Bathroom addition (upscale)	$107,251	$37,232	34.7%
Master suite addition (upscale)	$334,000	$80,734	24.2%

Source: Remodeling Magazine 2024 Cost vs. Value Report (U.S. average; actual ROI varies significantly by metro and price band)

Regional Cost: 300 sq ft Bedroom Addition with Slab Foundation

Same scope — 300 sq ft single-story bedroom, slab foundation, HVAC extension, mid-range finishes. The 2026 national average is about $45,000; the total runs roughly 60–65% higher in the costliest metros (New York, LA, Boston) than the lowest (Houston, Phoenix), driven mostly by labor and permit costs.

City	Low (total project)	Mid	High
Houston, TX	$30,000	$37,000	$52,000
Phoenix, AZ	$30,000	$38,000	$54,000
Atlanta, GA	$31,000	$39,000	$55,000
Dallas, TX	$31,000	$39,000	$55,000
Columbus, OH	$32,000	$41,000	$57,000
Miami, FL	$33,000	$42,000	$60,000
Chicago, IL	$36,000	$46,000	$65,000
Denver, CO	$37,000	$47,000	$66,000
Seattle, WA	$41,000	$52,000	$73,000
Boston, MA	$43,000	$54,000	$77,000
Los Angeles, CA	$45,000	$57,000	$80,000
New York, NY	$48,000	$61,000	$86,000

Source: Illustrative metro estimates informed by BLS construction-trade wages (SOC 47-2031, 47-2061) and published regional cost and permit-fee guides; figures are approximate and vary locally.

Build Out vs Build Up: The Foundation Math

The instinct says build up is cheaper because you skip foundation and roofing on the new floor. The math is more complicated.

Build out — adding ground-level square footage with its own foundation and roof — costs $130–$200 per sq ft for a bedroom. The foundation typically eats 10–15% of the budget, the roof another 5–8%. You can live in the existing house through construction.

Build up — adding a second story over part or all of the existing footprint — eliminates new foundation cost but requires three things that often add up to more: (1) a structural engineering analysis confirming the existing foundation can carry the additional load (typically $1,500–$5,000), (2) temporary removal of the existing roof and weather protection during framing (which often forces you to move out for 2–4 months and pay for temporary housing), and (3) commonly, retrofit of the existing foundation with helical piers or perimeter footings if the engineer's load analysis comes back short.

The practical rule: build out when you have lot space and your existing foundation is questionable. Build up when lot setbacks force you and the existing structure is post-1990 (code-compliant headers, modern foundation depth). For homes built pre-1960, build up almost always triggers foundation reinforcement that erases the per-sq-ft savings.

Why Sunrooms Are the Cheapest Addition (and What You Give Up)

A sunroom comes in at $110–$190 per sq ft because it skips four expensive systems: full insulation, year-round HVAC, code-compliant electrical for habitable rooms, and IRC-grade exterior wall framing.

The IRC (International Residential Code) distinguishes "habitable space" — which has to meet R305 minimum ceiling height (typically 7 ft for habitable rooms), R303 light and ventilation, and R310 emergency escape openings where sleeping rooms or basement-level habitable spaces are involved — from "sunroom," which IRC Section R301.2.1.1.1 categorizes by performance category and exempts from many full habitable-space requirements. This single classification difference cuts cost roughly in half.

What you give up: a sunroom doesn't count toward your home's heated square footage for resale, doesn't add a usable bedroom on the listing, can't be permanently insured as primary living space, and is unusable in summer 95°F+ humidity or winter sub-freezing temperatures. If you plan to use the space 12 months a year, the sunroom budget is false economy — pay the extra $50–$80 per sq ft for a full conversion that adds resale square footage.

The sweet spot: sunrooms are the right answer when you have a specific 6–9 month use (gardening, hobbies, dining in shoulder seasons) and don't need to count the space at resale.

In-Law Suite Economics: ADU Permitting and Tax Implications

An in-law suite — typically a bedroom, bathroom, and small kitchen/kitchenette as part of the main house — runs $180–$300 per sq ft because of the duplicated plumbing and electrical for the second kitchen.

If the suite has its own entrance and full kitchen, many jurisdictions reclassify it as an Accessory Dwelling Unit (ADU). This has three practical effects:

Permits get more expensive and slower. ADU permits in most states require separate plan review (often by the city planning department, not just building department) and frequently trigger requirements for an additional parking space, separate water/sewer hookups, or fire-separation between the units. Permit fees commonly run $2,000–$6,000 for an ADU vs $500–$2,000 for a regular addition.

Property taxes increase more. Most county assessors reassess additions that add bedrooms or living space, but ADUs trigger a larger reassessment because they're now a multi-family classification. Expect your annual property tax to increase by roughly the same percentage that the assessed value increases — for a $80,000 ADU on a $400,000 home, that's roughly 20% more property tax.

Resale and rental rules change. California, Oregon, and several other states have passed laws making ADUs explicitly rental-friendly. In those states, the in-law suite can pay for itself over 8–12 years as long-term rental income. In states without ADU-friendly legislation, short-term rental (Airbnb) is often prohibited and long-term tenant law applies.

If the addition is genuinely for family — not future rental — the cheaper path is to keep it as a "primary residence with extended living space" rather than a separately-permitted ADU.

Why Additions Take 3–6 Months Even for 300 Square Feet

Homeowners expecting a 4–6 week project are usually shocked. A typical 300 sq ft single-story addition runs 12–20 weeks from contract signing to final inspection. The breakdown:

Design and permit phase: 4–8 weeks. Drawings have to be prepared (often by an architect or design-build firm), submitted to the local building department, and reviewed. Larger jurisdictions (LA, NYC, Boston) routinely take 6–8 weeks just for plan review. This phase is often invisible to homeowners — work hasn't started yet, but the clock is running.

Foundation: 1–2 weeks. Excavation, footing inspection, pour, curing time (concrete needs 7 days to reach 75% strength before framing can sit on it).

Framing and roofing: 2–3 weeks. Walls, ceiling, roof structure. Roofing tie-in to the existing house often adds 3–5 days because the new roof has to be flashed into the existing one without leaks.

MEP rough-in: 1–2 weeks. Electrical, plumbing, HVAC duct runs. Then a rough-in inspection by the building department.

Insulation, drywall, finish: 3–5 weeks. Insulation inspection, then drywall (with mud/sanding/paint typically taking longer than homeowners expect), flooring, trim, fixtures.

Final inspection and punch list: 1–2 weeks.

Delays are normal — weather, material backorders (truss orders run 4–8 weeks lead time in 2026), permit revisions, and inspector scheduling. Plan a 20% time buffer into your expectations.

Addition Cost by Addition Size

Addition Size	Low	Average	High
150 sq ft	$14,048	$26,364	$39,019
225 sq ft	$19,058	$35,624	$52,500
300 sq ft	$23,978	$44,701	$65,700
450 sq ft	$33,654	$62,513	$91,575
600 sq ft	$43,200	$80,057	$117,038
900 sq ft	$62,068	$114,680	$167,250

National average at typical settings — use the calculator above for your exact inputs and location.

Planning a larger project? You may also want to estimate costs for cost to build a house, basement finishing cost, central ac installation cost, or foundation repair cost.

Cost basis & data provenance

This calculator is calibrated so a typical 300 sq ft bedroom, 1-story, slab, extend HVAC lands near a national average of ~$45,000 (~$150/sq ft), based on HomeGuide 2026; Angi 2026 (accessed 2026-06-16). Cost data is scheduled for review by 2027-01.

Sources

Costs are based on current industry ranges and vary by location and market conditions. See how we calculate costs — cost data last reviewed June 2026.