A house extension, usually called a home addition in the U.S., commonly costs $80,000-$240,000 for a basic 400-square-foot project, with an estimated average near $140,000. House extension cost depends most on floor area, foundation and roof work, room type, finish level, local labor rates, and whether plumbing or structural changes are needed.
| Item | Low | Average | High | Notes |
|---|---|---|---|---|
| 400-sq.-ft. ground-floor addition | $80,000 | $140,000 | $240,000 | $200-$600 per sq. ft.; basic to high-spec scope |
| 200-sq.-ft. bump-out | $30,000 | $55,000 | $100,000 | $150-$500 per sq. ft.; foundation and roof complexity matter |
| 400-sq.-ft. second-story addition | $120,000 | $220,000 | $360,000 | $300-$900 per sq. ft.; structural reinforcement may be required |
| Design, engineering, and permits | $5,000 | $15,000 | $35,000 | Often separate from the construction quote |
What a 400-square-foot home addition usually costs
A straightforward 400-square-foot ground-floor addition often falls around $80,000-$240,000, or roughly $200-$600 per square foot. A simple bedroom or family room with standard finishes may land near the lower end. A bathroom, kitchen, large openings into the existing house, or premium finishes can push the quote higher.
A practical planning figure is about $350 per square foot for a standard finished addition. That makes a 400-square-foot project approximately $140,000 before unusual site work or high-end upgrades. A small project may cost more per square foot because design, mobilization, permits, and utility connections do not shrink in proportion to floor area.
Assumptions: U.S. pricing, conventional wood framing, standard finishes, normal site access, and no major foundation or structural surprises. Local quotes can vary substantially. The estimate generally assumes a finished, weather-tight room connected to the home; it does not automatically include furniture, landscaping, temporary housing, or extensive repairs to older parts of the house.
Where framing, foundation, and labor appear in the quote
Contractor proposals may combine several trades into one construction price, so asking for line items helps identify what is included. These approximate shares describe a typical ground-floor addition; plumbing-heavy or architecturally complex work can shift the balance.
| Quote component | Typical range | Common pricing basis | What changes it | Usually included |
|---|---|---|---|---|
| Foundation and excavation | $12,000-$35,000 | $30-$90 per sq. ft. | Soil, slope, crawlspace or basement | Footings, slab or foundation wall |
| Framing, roof, and exterior shell | $25,000-$65,000 | $60-$165 per sq. ft. | Roof tie-in, siding, windows, spans | Framing, sheathing, weather barrier |
| Interior finishes and insulation | $20,000-$55,000 | $50-$140 per sq. ft. | Flooring, trim, drywall, finish grade | Insulation, drywall, paint, flooring allowance |
| Labor and project management | $25,000-$70,000 | $75-$125 per worker-hour | Trade availability and site complexity | Carpentry, installation, supervision |
| Design, permits, and engineering | $5,000-$35,000 | Flat fee or 5%-15% of construction | Review requirements and design scope | Plans, structural review, permit fees |
These ranges are not separate amounts to add blindly: a contractor’s per-square-foot quote may already include labor, materials, and overhead. Confirm whether the proposal covers demolition, disposal, utility extensions, inspections, taxes, and finish allowances.
For a comparable estimate, request the same written scope and allowances from each contractor. A low bid can omit costly items such as siding repairs, HVAC balancing, or the final connection between new and existing rooms.
How square footage, bathrooms, and foundations change the estimate
Size is only one pricing driver. A 250-square-foot room addition may cost $60,000-$150,000, while a 600-square-foot addition may cost $120,000-$360,000. The larger project generally has a lower cost per square foot when it repeats the same layout and finish level, but it still requires more materials and may need larger HVAC capacity.
Room use also matters. A bedroom or den without plumbing is usually less expensive than a bathroom or kitchen. Adding a full bathroom can add approximately $15,000-$40,000 for plumbing, fixtures, waterproofing, and ventilation; moving a kitchen or adding complex drainage may add $25,000-$75,000 or more, depending on distance and finish choices.
Foundation type and access can change excavation costs sharply. A level, accessible site with a slab may require less work than a steep lot, tight urban side yard, or addition over a basement. For a second story, an engineer must assess the existing foundation and framing. Reinforcement can add $20,000-$80,000, and major structural limitations may make the project impractical at the initial budget.
Get an early structural review before relying on a per-square-foot estimate. A 10-foot plumbing run is typically simpler than a 40-foot run through finished rooms; likewise, an uncomplicated roof connection is less costly than a valley, multiple roof planes, or extensive matching of older exterior materials.
Can a bump-out or second story cost less than expanding outward?
A bump-out enlarges one room without creating a full new wing. Small bump-outs often cost $30,000-$100,000, but a compact project can reach $150-$500 per square foot because it still needs design, structural work, exterior finishing, and a roof connection. A shallow projection may avoid a full foundation, though cantilever limits and engineering requirements apply.
A second-story addition can preserve yard space, but typical pricing is higher: roughly $300-$900 per square foot, or $120,000-$360,000 for 400 square feet. The estimate can include roof removal, temporary weather protection, stairs, structural reinforcement, and new mechanical systems. A ground-floor addition is often less disruptive structurally, but may require excavation and sacrifice outdoor space.
Converting an existing garage or attic can cost less than building new space if the structure is sound and has adequate ceiling height. A garage conversion may run $20,000-$75,000 for a basic finished room, while adding plumbing, insulation, windows, or a new foundation can raise the total. It may not provide the same parking or storage value as an extension.
Compare the total project scope, not just the advertised price per square foot. A conversion quote may exclude structural upgrades or utility work that a new addition includes, and local rules may restrict changes to garages, lot coverage, or building height.
Why metro labor rates and permit rules shift addition prices
Construction costs in high-demand coastal metros and major cities often run 15%-35% above a broad national planning estimate. Expensive labor markets, limited contractor availability, complex permitting, and difficult material delivery all contribute. Smaller cities and rural areas can be 5%-20% below typical metro pricing, although remote travel or limited trade availability may erase some savings.
Regional percentages are planning ranges, not fixed surcharges. A suburban project with easy access may be less costly than a downtown project in the same metro area, while a rural house requiring a long utility extension may cost more than an in-town job. Ask contractors to price the same drawings and finish allowances to make local comparisons meaningful.
Permit charges vary by city and project value. Plan review, building permits, inspections, and impact fees can total about $1,000-$10,000 for a modest addition, but larger projects or strict jurisdictions can exceed that. Design and engineering fees are separate in many proposals. Permit requirements may also trigger energy-code upgrades or changes to existing electrical, smoke-alarm, or structural conditions.
Confirm local permit and utility fees before treating a contractor’s construction figure as the full budget. Set aside a contingency of about 10%-20% for concealed conditions, especially in older homes where framing, wiring, or drainage may not match original plans.
How many weeks of labor should the schedule include?
A modest bump-out may take 6-12 weeks of on-site construction after plans and permits are approved. A typical 400-square-foot addition often needs 3-6 months of construction, while a second-story project may take 5-9 months. Design, permitting, contractor scheduling, and material lead times can add several months before work begins.
Crews may include a general contractor, carpenter, electrician, plumber, HVAC technician, roofer, and finish trades. A project with several specialty trades or limited access can have gaps between work phases. Hourly labor rates for skilled trades commonly fall around $75-$125, with higher rates in expensive markets or for emergency and specialty work.
Construction duration affects the budget through supervision, site protection, temporary utilities, and possible lodging costs if the home cannot be occupied. Occupants may also need to budget for dust control or temporary access changes. Ask whether the estimate includes a schedule, cleanup, protection of occupied rooms, and charges for delays caused by owner selections or unavailable materials.
Allow for both construction time and the preconstruction queue. A firm start date is more reliable when the plans, permit status, major selections, and material orders are settled before the contract’s mobilization date.
Which removal, utility, and finish charges are easy to miss?
Demolition and disposal may add $2,000-$10,000 for straightforward removal, with higher charges if the project includes a large deck, masonry, hazardous materials, or difficult hauling. A contractor may price a dumpster separately at $500-$1,500 per load. Ask whether the quote includes removal of landscaping, fences, or paving affected by equipment access.
Utility work is another common variable. Extending electrical circuits may cost $1,000-$5,000 for a limited scope; a new HVAC zone or equipment upgrade can add $3,000-$12,000. Plumbing connections can vary widely with fixture count, pipe route, and drain access. If the existing electrical panel, furnace, or water heater lacks capacity, replacement may be needed beyond the addition’s basic allowance.
Finish allowances can make two bids appear similar while covering different quality levels. Window, cabinet, flooring, tile, lighting, and plumbing-fixture budgets should list a dollar allowance or product grade. A $5,000 flooring allowance may not cover a premium material and installation throughout a large room, and owner-selected upgrades can raise the final invoice.
Ask for exclusions and allowance amounts in writing before signing. A clear contract should state disposal, utility connections, weather protection, cleanup, inspection fees, and how change orders are priced.
Three addition estimates show how scope changes the total
The examples below use illustrative U.S. contractor pricing and standard-to-midrange finishes. They include typical construction and project labor but assume normal access; unusually high local rates, concealed damage, or extensive landscaping work can increase the totals.
| Project example | Scope and unit rate | Labor assumption | Estimated total |
|---|---|---|---|
| Bedroom addition | 300 sq. ft. at $250-$375 per sq. ft.; no bathroom | 3-4 workers, about 800-1,100 hours | $75,000-$112,500 |
| Family room with bathroom | 400 sq. ft. at $350-$550 per sq. ft.; one full bath | 4-6 workers, about 1,400-1,900 hours | $140,000-$220,000 |
| Second-story bedroom suite | 500 sq. ft. at $450-$750 per sq. ft.; structural review and bathroom | 5-7 workers, about 2,000-2,800 hours | $225,000-$375,000 |
The first example is relatively simple because it avoids new plumbing. The family room example carries a bathroom premium for drainage, fixtures, ventilation, and finish work. The second-story estimate reflects roof removal, stair construction, and potential reinforcement as well as the room itself.
Use these examples as scope comparisons, not bids for a specific address. Obtain a site visit and written proposal before making financing decisions, and check that each estimate includes the same project size, finish allowances, structural work, and permit responsibilities.
How to lower a house extension estimate without cutting essentials
Controlling the footprint is often the most direct way to control the total. Keep the new room’s shape simple, align exterior walls where possible, and avoid unnecessary roof valleys or long plumbing routes. Reducing floor area by 50 square feet at $300-$500 per square foot can trim roughly $15,000-$25,000, although fixed design and permit fees may remain.
Choose standard, readily available windows, doors, and finishes, and finalize selections before ordering. Keeping plumbing near existing supply and drain lines can avoid costly rerouting. If the project has multiple phases, ask whether completing related work together reduces repeated mobilization, but do not bundle unrelated upgrades simply to qualify for a discount.
Prepare the site where safe and permitted: clear personal items, decide what landscaping can be removed, and provide accurate information about known utilities or prior repairs. Compare at least three detailed bids, checking licensing, insurance, references, exclusions, and payment schedules. A quote that is substantially below others may omit necessary work rather than reflect a genuine saving.
Protect structural safety, weatherproofing, and required inspections when trimming the budget. Consider a smaller addition or a well-planned conversion before reducing insulation, drainage, or engineering; correcting those items later can cost more than including them in the original scope.