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.

Estimated Cost

Location

Low

$25,795

Average

$47,270

High

$82,583

Project Details

Adjust the project scope below to update the estimate instantly.

Preparing interactive controls…

Addition Size(sq ft)

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

Room Type

Addition Location

For a build-up, the selected foundation type is ignored and structural investigation, reinforcement, roof removal, and stairs are priced separately

Foundation Type

HVAC

Scope Notes & Exclusions3 planning notes included with this estimate

Room type selects explicit room, flooring, plumbing, glazing, and finish assemblies; it does not multiply the whole project.
Warning: An HVAC extension assumes a load calculation confirms that the existing equipment and ducts have capacity for the addition.
Excluded: Land purchase, major site clearing, utility-service or panel upgrades, surveys, zoning variances, hazardous-material abatement, temporary housing, and landscaping

Cost Breakdown

Review the subtotals first, then expand a section for itemized quantities and price ranges.

Materials & Equipment13 line items · 26% of the average estimateAverage$12,281

Item	Qty	Low	Average	High
Footings, Reinforcing, Concrete & Slab Materials	300 sq ft footprint	$1,500	$2,400	$3,600
Wall, Floor / Ceiling and Roof Framing Materials	300 sq ft added floor	$2,400	$3,600	$5,400
Architectural Asphalt Shingle Squares	400 sq ft purchase area	$600	$880	$1,280
Starter, Ridge Caps, Flashing, Drip Edge & Fasteners	330 sq ft roof deck	$83	$149	$248
Underlayment / Ice-Barrier Allowance	330 sq ft roof deck	$50	$83	$149
Branch Wiring, Receptacles, Lighting and Alarms	300 sq ft added floor	$450	$750	$1,200
Wall and Ceiling Insulation / Air-Sealing Materials	930 sq ft envelope	$419	$698	$1,116
Drywall, Fasteners and Joint Materials	930 sq ft wall / ceiling	$326	$465	$744
Primer, Paint and Consumables	930 sq ft painted surface	$140	$233	$465
Carpet (including 10% cutting / seam allowance)	330 sq ft	$330	$990	$2,310
Carpet Cushion, Tack Strip, Seam Tape & Consumables	330 sq ft purchase area	$165	$413	$825
Carpet Transitions, Tack Strip & Doorway Details	1 rooms	$50	$120	$200
Duct, Register, Return and Control Materials	300 sq ft served	$900	$1,500	$2,400
Subtotal		$7,413	$12,281	$19,937

Installed Assemblies & Mixed Scope4 line items · 25% of the average estimateAverage$11,780

Item	Qty	Low	Average	High
Existing Roof Tie-In, Flashing & Drainage Detailing	1 job	$1,200	$2,200	$4,000
Base Installed Matching Vinyl Siding Assembly	504 sq ft net wall	$1,512	$3,780	$6,048
Code-Compliant Windows and Exterior Door Allowance	1 job	$1,800	$3,000	$5,500
Opening, Header, Protection & Interior Connection to Existing House	1 job	$1,500	$2,800	$5,000
Subtotal		$6,012	$11,780	$20,548

Labor, Trade Services & Removal9 line items · 27% of the average estimateAverage$12,795

Item	Qty	Low	Average	High
Excavation, Forms, Pour & Slab Finishing	300 sq ft footprint	$1,200	$2,100	$3,000
Structural Framing and Sheathing Labor	300 sq ft added floor	$2,700	$4,500	$7,200
Architectural Shingle Layout & Installation Labor	330 sq ft roof deck	$594	$792	$1,122
Electrical Rough-In, Trim, Testing and Panel Connection	300 sq ft added floor	$750	$1,350	$2,100
Insulation and Air-Sealing Labor	930 sq ft envelope	$326	$558	$930
Drywall Hanging and Finishing Labor	930 sq ft wall / ceiling	$605	$930	$1,395
Preparation, Priming and Painting Labor	930 sq ft painted surface	$279	$465	$837
Carpet Layout, Seaming, Stretching & Installation Labor	300 sq ft floor area	$300	$600	$1,200
HVAC Extension, Airflow Setup and Testing	300 sq ft served	$900	$1,500	$2,400
Subtotal		$7,654	$12,795	$20,184

Design, Permits & Project Costs3 line items · 22% of the average estimateAverage$10,414

Item	Qty	Low	Average	High
Architectural Design, Engineering & Documentation	1 allowance	$1,054	$2,580	$6,067
Building and Trade Permit / Inspection Allowance	1 job	$500	$1,200	$2,500
General Contractor Overhead & Profit	1 allowance	$3,162	$6,634	$13,347
Subtotal		$4,716	$10,414	$21,914

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

The model derives a planning perimeter from floor area, then keeps footprint, gross wall, net cladding, roof-deck, roofing-purchase, drywall/paint, and flooring-purchase quantities separate. A room type selects a real assembly instead of multiplying the whole project: bedrooms use carpet, family rooms add an open-span/header and glazing allowance, three-season sunrooms use a glazed-wall package without year-round insulation, drywall, or HVAC, and in-law suites add bathroom, kitchenette, and MEP scope. Ground-level additions use the selected foundation; second-story additions replace it with structural investigation, reinforcement, roof removal/weather protection, and a stair assembly. Design, permits, and GC overhead remain separate and recompute from applicable construction lines.

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 ground-level bedroom addition on a slab foundation with extended HVAC, computed directly from the canonical component model and rounded to the nearest $100. Room types are separate assemblies, so do not apply a generic multiplier to these rows.

Dimensions	Floor Area	Low	Average	High
10 × 12	120 sq ft	$15,400	$28,500	$50,200
10 × 20	200 sq ft	$20,600	$37,700	$65,500
12 × 16	192 sq ft	$20,100	$36,900	$64,100
12 × 20	240 sq ft	$23,000	$42,000	$72,700
12 × 24	288 sq ft	$26,100	$47,400	$81,700
16 × 20	320 sq ft	$28,000	$50,800	$87,400
20 × 20	400 sq ft	$32,800	$59,400	$101,800

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, different assemblies. These rows are direct outputs for 300 sq ft on a slab. The three-season sunroom omits year-round insulation, drywall, and HVAC; the in-law suite adds a bathroom, kitchenette, and their plumbing/electrical scope.

Addition Type	Cost Range ($/sq ft)	Mid Total (300 sq ft)	What Drives Cost
Sunroom (3-season)	$76 – $262 modeled	$42,915	Glazed-wall package; no permanent HVAC or full habitable-room finish
Bedroom (no plumbing)	$86 – $275 modeled	$47,270	Slab, full envelope, carpet, electrical, HVAC extension
Family room (open span)	$100 – $317 modeled	$54,813	LVP, wider opening/header, added glazing and trim
In-law suite (kitchen + bath)	$156 – $520 modeled	$87,313	Bathroom, kitchenette, plumbing and appliance connections
Second-story bedroom	$125 – $412 modeled	$69,693	Engineering, reinforcement, roof removal/weather protection and stairs

Source: Computed directly from this calculator's canonical model. The wide low/high span reflects assembly and market uncertainty; an engineer and local bids are required for a second-story scope.

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 ground-level bedroom, slab foundation, HVAC extension, and standard finishes. The canonical national midpoint is about $48,800; illustrative metro totals vary mainly with labor and material factors, while the permit allowance stays local and unscaled.

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.

Two Home Addition Cost Examples

Example 1 — a 300 sq ft ground-level bedroom on a slab with an extension of the existing HVAC system. The model derives a 70-ft planning perimeter, 630 sq ft of gross wall, 555 sq ft of siding purchase area, 330 sq ft of sloped roof deck, four roofing squares, 930 sq ft of drywall/paint surface, and 330 sq ft of carpet purchase area. It returns $25,795–$82,583 with a $47,270 midpoint after correcting the shared siding benchmark so installed labor is counted once. This remains inside the cited $20,000–$60,000 bedroom-project comparison band at the midpoint, while its high case captures premium and difficult-market exposure.

The in-law-suite option adds bathroom, kitchenette, plumbing/appliance connections, and dedicated equipment. Zoning or ADU requirements, utility upgrades, and separate service connections remain outside the modeled allowance; use the live result when changing these inputs rather than relying on a static example.

Build Out vs Build Up: The Foundation Math

The instinct says build up is cheaper because it avoids a new ground-level footprint. The scope comparison is more complicated.

Build out adds a new foundation, exterior walls, and roof. It may be easier to sequence when the lot, setbacks, drainage, and existing-house connection allow it, but it consumes site area and creates new excavation and roof-tie-in work.

Build up removes most new-footprint work but adds investigation of the existing foundation, walls, beams, and lateral-load path; roof removal and temporary weather protection; a stair and altered floor framing; and possible strengthening identified by the design. Temporary relocation and protection needs depend on the contractor's sequence and the extent of work.

There is no reliable construction-year cutoff that proves a house can accept another story. Compare build-out and build-up concepts only after a qualified designer has reviewed the existing structure, zoning envelope, egress, utilities, and site constraints.

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

A three-season sunroom can cost less than a conventional room because it uses a glazed-wall enclosure and reduced framing/electrical scope instead of a complete year-round insulation, drywall, paint, and HVAC assembly. The model still prices its slab, roof, roof tie-in, exterior connection, glazing, tile floor, basic electrical, design, permits, and GC scope; it does not treat the space as a cheap shell.

The IRC (International Residential Code) distinguishes habitable space — which has requirements for ceiling height, light, ventilation, energy performance, and emergency escape where applicable — from defined sunroom categories. The exact classification and structural, glazing, electrical, and energy-code requirements are local design questions, not something this calculator certifies.

What you give up is year-round conditioned-room scope. If you plan to heat and cool the space permanently or count it as habitable square footage, use a conventional bedroom or family-room assembly and have the designer specify the envelope and HVAC load. A three-season budget is appropriate only when the intended use and local approval match that classification.

In-Law Suite Economics: ADU Permitting and Tax Implications

An in-law suite typically combines sleeping space with a bathroom and some food-preparation facilities, which is why its modeled plumbing, electrical, cabinetry, appliance, and ventilation scope is higher than a bedroom addition.

Whether the project is legally an accessory dwelling unit depends on local definitions and the approved design—not the label used by the owner or contractor. A separate entrance, cooking facilities, fire separation, parking, utility metering, owner-occupancy rules, impact fees, and rental permissions may matter, but requirements vary substantially by jurisdiction. Ask planning, building, utility, and fire-review staff which rules apply before finalizing the layout.

Tax assessment and rental treatment also depend on local law and the completed use. Do not estimate a property-tax increase as a fixed percentage of construction cost, and do not assume rental income or a payback period without local rent, vacancy, operating-cost, financing, and tax analysis. This calculator excludes those legal and financial outcomes.

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

A small addition is a multi-stage project, and calendar duration varies more than floor area alone suggests. A planning sequence commonly includes existing-condition survey and design, zoning/building review, procurement, excavation and foundation work, framing and dry-in, trade rough-ins and inspections, insulation and finishes, and final approvals.

Concrete placement and the start of framing must follow the project specifications, mix design, weather conditions, and the designer's or contractor's required strength and curing sequence; a universal seven-day framing rule is not appropriate. Roof tie-in, temporary weather protection, utility shutdowns, and occupied-home protection can also control the schedule.

Ask bidders for a project-specific schedule showing design and permit assumptions, long-lead items, inspection hold points, owner selections, weather allowances, and the work required before the addition is secure and usable. Treat generic month ranges as early planning context, not a promised completion date.

Addition Cost by Addition Size

Addition Size	Low	Average	High
150 sq ft	$16,580	$30,824	$54,826
225 sq ft	$21,213	$39,105	$68,805
300 sq ft	$25,795	$47,270	$82,583
450 sq ft	$34,702	$63,068	$109,240
600 sq ft	$43,514	$78,652	$135,532
900 sq ft	$60,754	$109,108	$186,944

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

Scope-matched

This calculator checks a typical 300 sq ft bedroom, 1-story, slab, extend HVAC against a scope-matched benchmark of ~$45,000 (~$150/sq ft), using HomeGuide 2026; Angi 2026 (accessed 2026-06-16). Cost data is scheduled for review by 2027-01.

HomeGuide — Home Addition Cost: $80–$300+ per sq ft across addition types. The bedroom default uses the lower-middle portion of the broad type range and includes a slab and HVAC extension. Checked 2026-06-16.
Angi — Cost to Add a Room: $20,000–$60,000 for a bedroom addition. The default 300-sq-ft bedroom sits within this room-specific project range. Checked 2026-06-16.

Sources

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

Frequently Asked Questions

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 avoids a new ground-level foundation, but it still requires structural engineering, removal and temporary protection of the existing roof, a new roof over the added floor, stairs, and often reinforcement. This calculator prices those assemblies separately instead of assuming a second story is automatically cheaper. Build out is usually easier to sequence when lot setbacks allow it.

Do I need a permit for a home addition?

Most structural home additions require building approval, and electrical, plumbing, or mechanical work may require separate trade permits. Requirements and fees are set locally. This calculator carries a $500–$2,500 planning allowance for permits and inspections, separate from design and engineering; confirm the actual approval path with the authority having jurisdiction before budgeting.

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?

An addition can provide useful space, but resale recovery varies by project type, specification, location, and market timing. The cited national Cost vs. Value examples on this page show that additions commonly recover only part of project cost. Compare the same-scope construction budget with moving costs and obtain local valuation advice instead of assuming a fixed return.

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.