A house extension in the U.S. typically costs about $2,150-$5,380 per square metre, or roughly $200-$500 per square foot, with a national planning average near $3,230 per square metre. Foundation work, number of stories, finish level, plumbing, and local labor rates can move a quote well outside that range.
| Item | Low | Average | High | Notes |
|---|---|---|---|---|
| Finished extension per sq m | $1,500 | $3,230 | $5,920 | About $140-$550 per sq ft |
| 300-sq-ft extension | $42,000 | $96,900 | $165,000 | About 27.9 sq m |
| 600-sq-ft extension | $84,000 | $193,800 | $330,000 | About 55.7 sq m |
| Design, engineering, and permits | $3,000 | $12,000 | $30,000+ | May be quoted separately |
Typical house extension cost per square metre
For a conventional, finished ground-floor addition, a practical budget range is $2,150-$5,380 per sq m ($200-$500 per sq ft). A straightforward project using standard finishes may fall near the lower end; complex access, higher-cost metro labor, custom glazing, or extensive interior matching can push the price upward. A small addition can cost more per square metre than a large one because design, mobilization, and utility work are spread over less floor area.
For early budgeting, use approximately $3,230 per sq m ($300 per sq ft), then add design fees and a contingency if they are not included in the builder’s quote.
These figures assume a permanent, enclosed addition connected to an existing home, with a foundation, roof, insulation, electrical work, and ordinary interior finishes. They are not a direct price for a porch, deck, garage, or unfinished shell. Local code requirements and the work needed to open the existing exterior wall can change the scope.
Assumptions: U.S. national planning ranges, standard-grade materials, typical site access, and a completed living space. One square metre equals about 10.764 square feet.
Where foundation, framing, and finishes enter the quote
A contractor’s estimate may combine work into one lump sum, but the cost usually comes from several trade groups. The table gives broad allowances for a standard addition; individual bids can shift costs between categories depending on whether design, overhead, and cleanup are included.
| Cost component | Typical range | Common pricing basis | What changes the charge |
|---|---|---|---|
| Foundation and site preparation | $270-$860 per sq m | $25-$80 per sq ft | Excavation, soil, drainage, and foundation type |
| Framing, roof, and exterior shell | $430-$1,080 per sq m | $40-$100 per sq ft | Span, roof tie-in, siding, and window count |
| Electrical, plumbing, and HVAC | $215-$860 per sq m | $20-$80 per sq ft | New bathroom or kitchen, panel capacity, duct runs |
| Interior insulation and finishes | $430-$1,290 per sq m | $40-$120 per sq ft | Flooring, cabinets, trim, paint, and finish grade |
| Labor, project management, and overhead | $540-$1,290 per sq m | $50-$120 per sq ft | Crew rates, duration, site access, and contractor structure |
| Permits, delivery, and disposal | $3,000-$15,000+ total | Project allowance | Jurisdiction, hauling, inspections, and demolition |
These component ranges are planning allowances, not amounts to add automatically to a per-square-foot turnkey quote. The builder may already include labor, overhead, permits, or waste removal in the total. Ask each contractor to identify inclusions and exclusions before comparing the headline price.
How floor area, stories, and plumbing change the rate
Size and design affect both the total price and the rate per square metre. A 150-sq-ft (13.9-sq-m) bump-out may cost $250-$600 per sq ft because it still needs design, foundation work, and utility connections. A simple 500-sq-ft (46.5-sq-m) room addition might land around $200-$400 per sq ft, depending on location and finish choices.
Adding a second story commonly costs more per square foot than building outward. Structural reinforcement, stairs, roof removal, and protection from weather can bring the total to roughly $300-$600 per sq ft. The exact premium depends on the existing house’s framing and whether occupants remain in the home during construction.
Plumbing is another major price trigger. A bedroom or family room addition without new plumbing is generally less expensive than a bathroom or kitchen extension. A new bathroom can add about $15,000-$40,000 or more, especially when drains must travel a long distance or the existing septic system needs work.
Foundation conditions also matter. A slab on an uncomplicated, level site may be less costly than a crawlspace or basement. Sloped ground, poor soil, retaining walls, or difficult equipment access can add thousands of dollars. Get a site-specific foundation allowance rather than assuming a flat per-square-foot rate covers every lot.
Labor example: 500-1,000 combined crew hours at $50-$100 per hour represents $25,000-$100,000 in direct labor before contractor overhead and other project costs.
Lower the extension price by controlling scope and finishes
Keeping the footprint compact and rectangular can reduce foundation, roof, and framing complexity. Avoiding multiple corners, unusual roof valleys, and long structural spans can make the design easier to build. A change from a 300-sq-ft plan to 400 sq ft increases floor area by one-third, but total project cost may rise by more or less depending on fixed fees and the layout.
Keep bathrooms and kitchens close to existing plumbing where the layout allows. Moving drains or extending water and vent lines can cost more than choosing a modestly priced sink or faucet. Before final drawings, ask the designer or builder to price the plumbing location alternatives.
Standard windows, stock cabinets, and widely available siding can help control material and installation costs. Custom windows, premium tile, specialty cladding, and bespoke millwork tend to increase both purchase prices and labor time. Specify the finish level in writing so bids are pricing the same materials.
Compare at least three itemized bids using the same plans and allowances. Check whether each includes demolition, structural engineering, permit fees, utility connections, cleanup, and tax where applicable. Resolve unclear allowances before signing, because a low initial quote can become expensive through exclusions and change orders.
Regional labor rates shift the same extension plan
Construction prices vary substantially by metro area. For a similar scope, high-cost coastal cities and some dense urban markets can run about 15%-35% above a national midpoint. Many mid-cost suburban markets may sit near the midpoint, while some lower-cost rural areas can be 10%-20% below it. These comparisons are broad; local material availability and contractor demand can reverse the pattern.
In practical terms, a 400-sq-ft (37.2-sq-m) standard addition priced at $300 per sq ft would total $120,000 before any excluded professional fees. A 20% regional increase would bring the same base scope to about $144,000; a 15% decrease would put it near $102,000. These are illustrative adjustments, not guaranteed local quotes.
Urban lots may add costs for limited staging, street permits, parking restrictions, or hand-carrying materials. Rural sites can involve longer contractor travel, well or septic work, and extra utility trenching. Request quotes from builders who regularly work in the same municipality and can account for its permit and access conditions.
How long a 300- to 600-square-foot addition takes
A modest addition often needs several weeks for design and permit review before construction begins. Once approved, a straightforward 300- to 600-sq-ft build may take roughly 3-6 months on site. Larger additions, second-story projects, complex utility work, or weather delays can extend the schedule to 6-9 months or longer.
Time affects cost through labor, supervision, temporary protection, and the number of visits by specialty trades. A longer schedule does not always mean a higher final price if the contractor’s bid is fixed, but delays caused by owner changes or concealed site conditions can generate added charges. Ask how the contract treats extensions, escalation, and change orders.
Many contractors bill labor through a fixed project price rather than an hourly invoice. For planning, trade labor can represent a substantial share of the total; local rates may run roughly $50-$100 per worker-hour, with licensed specialists often higher. Get a written schedule showing major milestones, payment triggers, and the process for approving extra work.
Three extension quotes for different scopes
The examples below illustrate how scope changes the total. Each assumes ordinary access and midrange U.S. pricing, with design and permit allowances shown separately where relevant. Real quotes depend on local labor, existing-home conditions, and finish selections.
| Project | Area and rate | Estimated construction total | Other budget items |
|---|---|---|---|
| Basic bedroom addition | 250 sq ft at $240-$340 per sq ft | $60,000-$85,000 | $4,000-$10,000 design and permits |
| Family room with upgraded glazing | 400 sq ft at $280-$420 per sq ft | $112,000-$168,000 | $6,000-$15,000 design, permits, and site work |
| Bedroom and bathroom addition | 600 sq ft at $350-$550 per sq ft | $210,000-$330,000 | $8,000-$20,000+ design, permits, and utility work |
The first example is approximately 23.2 sq m, with a construction rate of about $2,580-$3,660 per sq m. The family room is about 37.2 sq m, or $3,010-$4,520 per sq m. The bedroom-and-bath project is about 55.7 sq m, or $3,770-$5,920 per sq m. These conversions help compare the requested metric rate with U.S. contractor quotes usually stated per square foot.
Compare the scope alongside the rate: the bathroom example costs more not simply because it is larger, but because it includes plumbing and more demanding finishes.
Permits, utility work, and removal that may sit outside the bid
Permit charges vary by city and project valuation. A straightforward addition may involve several hundred to a few thousand dollars in building permits, while plan review, structural review, impact fees, and multiple inspections can raise the amount. Some jurisdictions calculate charges as a percentage of declared construction value; others use fixed or area-based fees.
Expect separate professional costs if the builder’s price excludes architectural plans, structural calculations, surveying, or energy documentation. A combined design and engineering budget may be around $3,000-$15,000 for a modest addition and $15,000-$30,000 or more for a complicated design. Verify whether permit drawings and revisions are included.
Demolition and disposal may add about $1,000-$8,000 for ordinary removal, but extensive demolition, asbestos or lead-safe procedures, or difficult hauling can cost considerably more. Utility upgrades can also be material: an electrical panel replacement may cost roughly $1,500-$4,000, while a sewer, septic, or major HVAC upgrade can exceed that range.
Ask the contractor to state who obtains permits and whether inspections, fees, delivery, and debris removal are included. Keep a 10%-20% contingency for concealed conditions, especially in older homes, but do not treat it as a substitute for a detailed bid.
Extension versus conversion for the same added space
A garage conversion can provide finished area without building a new foundation and roof, often at about $80-$200 per sq ft ($860-$2,150 per sq m). Insulation, windows, flooring, heating, and code-required egress can raise the price. The existing slab, ceiling height, moisture control, and parking needs determine whether conversion is practical.
Finishing an attic may cost around $50-$150 per sq ft ($540-$1,615 per sq m) when access and headroom are suitable. Adding stairs, dormers, structural support, or roof work can push the price higher. A basement finish may range around $30-$100 per sq ft ($320-$1,080 per sq m), excluding major waterproofing or foundation repair.
These alternatives usually cost less per square foot than a full ground-up addition because they reuse part of the home’s shell. However, they may not deliver the same daylight, layout, ceiling height, or direct outdoor access. Compare the cost of usable, code-compliant finished area rather than comparing headline square-foot prices alone.