A single storey extension, usually called a single-story home addition in the U.S., typically costs $45,000-$180,000, with many projects averaging about $105,000. Total price depends most on square footage, foundation and roof work, finishes, utility connections, and local labor rates.
| Item | Low | Average | High | Notes |
|---|---|---|---|---|
| Finished addition | $45,000 | $105,000 | $180,000 | About 300-600 sq ft at $150-$300 per sq ft |
| Basic shell | $30,000 | $60,000 | $105,000 | Foundation, framing, roof, and exterior enclosure |
| Kitchen or bathroom addition | $70,000 | $145,000 | $250,000 | Higher plumbing, electrical, and finish costs |
| Design and permits | $3,000 | $9,000 | $20,000 | Varies with drawings, engineering, and jurisdiction |
What a 300- to 600-Square-Foot Addition Usually Costs
Most finished single-story additions land around $150-$300 per square foot, or $45,000-$180,000 for a 300- to 600-square-foot project. A straightforward bedroom or family room with standard finishes may fall near $150-$225 per sq ft. A kitchen, bathroom, or addition requiring extensive structural changes can reach $250-$400 per sq ft.
For a practical midpoint, a 450-square-foot room addition at $225 per sq ft comes to about $101,250 before unusual site work or high-end upgrades. That estimate assumes a conventional foundation, standard windows and siding, average interior finishes, and normal access for a local contractor. Connecting the new space to existing heating, cooling, plumbing, and electrical systems can shift the total considerably.
Costs generally include foundation, framing, roofing, exterior walls, windows, basic interior finish, and contractor labor. They may not include major landscaping, moving an electrical panel, replacing an undersized HVAC system, or temporary housing. Assumptions: U.S. pricing, standard materials, ordinary site access, and a permitted addition attached to an existing house.
How Foundation, Framing, and Finish Work Shape the Quote
Labor and structural work usually account for the largest share, while plumbing and premium finishes create the widest project-to-project swings. These planning ranges show typical portions of a finished 450-square-foot addition; exact bids may group costs differently.
| Work item | Typical price | Unit or scope | What changes the charge |
|---|---|---|---|
| Foundation and excavation | $8,000-$25,000 | $18-$55 per sq ft | Soil, slope, slab versus crawl space |
| Framing and roof | $18,000-$42,000 | $40-$95 per sq ft | Roof tie-in, spans, structural beams |
| Exterior doors, windows, siding | $8,000-$24,000 | $18-$55 per sq ft | Window count, material, matching existing exterior |
| Electrical and HVAC | $7,000-$22,000 | $15-$50 per sq ft | New circuits, ducts, equipment capacity |
| Interior finish and paint | $12,000-$35,000 | $27-$80 per sq ft | Flooring, insulation, trim, finish grade |
| Design, permits, and inspections | $3,000-$15,000 | Project allowance | Drawings, engineering, municipal fees |
Some line items overlap because contractors may price a complete trade package rather than separate materials and installation. General contractor overhead and profit are often included in the bid, commonly adding roughly 10%-20% across subcontracted work. A contingency of 10%-15% is prudent for concealed damage or changes discovered after demolition.
Labor can be estimated with . For example, 500-800 combined crew hours at $50-$100 per hour represents $25,000-$80,000 in labor value, though contractor quotes usually bundle labor with materials and project management.
Which Square-Footage and Utility Choices Raise the Price?
Size, room function, foundation conditions, and system capacity are the clearest reasons two additions of similar area receive different bids. A simple 300-square-foot room may cost $45,000-$90,000, while a 600-square-foot addition can cost $90,000-$180,000. Larger projects cost more overall but may have a lower rate per square foot when kitchens, bathrooms, and mechanical connections are not added.
Room use matters. A bedroom or living room with no new plumbing may fit $150-$250 per sq ft. A full kitchen often reaches $250-$400 per sq ft because cabinets, counters, appliance circuits, ventilation, and plumbing add scope. A bathroom addition can add $15,000-$40,000 or more, especially when drains must run far from existing plumbing.
Foundation and roof geometry create additional thresholds. A level, accessible lot with a slab may add roughly $18-$35 per sq ft for foundation work; steep grades, poor soil, retaining walls, or crawl-space construction can raise that to $35-$70 per sq ft. A simple roof tie-in may be manageable, but a complex roof intersection or long structural span can add $5,000-$20,000.
Existing equipment must also have capacity. If an HVAC system cannot handle the added area, a ductless mini-split may cost $3,000-$8,000 installed, while a larger central system or substantial duct alterations can cost $7,000-$18,000. Electrical service upgrades can add $2,000-$6,000 or more when the current panel lacks space or capacity.
How Regional Labor Rates Change Addition Prices
Local labor markets and permitting rules can move the same plan 10%-30% above or below a national midrange. In lower-cost rural and smaller-market areas, straightforward additions may run $130-$220 per sq ft. Major metro areas in the Northeast, West Coast, and parts of the Mountain West commonly reach $220-$400 per sq ft, particularly where trades are busy or site access is tight.
These are broad comparisons, not fixed regional price lists. A rural site may have lower hourly rates but higher delivery charges or fewer available contractors. Urban sites may require parking permits, debris handling, neighbor protection, and restricted work hours. Coastal wind, wildfire, snow-load, or seismic requirements can also increase engineering and material specifications.
Permitting costs vary widely. A basic municipal permit package might total $1,000-$5,000, while plans requiring structural engineering, energy documentation, impact fees, or multiple reviews can reach $8,000-$20,000. Before comparing contractor totals, check whether design, permit fees, utility work, and inspection corrections are included or listed separately.
How Long Construction Takes and What Labor Covers
Many 300- to 600-square-foot additions take 3-6 months from design through final inspection, with construction itself often lasting 10-20 weeks. A simple room on a prepared site may finish faster; a kitchen or bathroom addition with structural connections and inspections often takes longer. Design and permit review can add several weeks or months before a crew starts.
A typical crew may include a project manager, carpenter, electrician, plumber, HVAC technician, roofer, and finish trades. Not all workers are on site at once. Regional contractor rates often fall around $50-$100 per hour for skilled labor, while specialized or emergency work may cost more. Weather delays, backordered windows, and inspection scheduling can stretch the calendar without changing the original labor rate.
Quotes should identify whether site supervision, cleanup, temporary weather protection, and final punch-list work are included. Demolition or opening an exterior wall may require temporary supports and careful weatherproofing. If the household remains in the home, dust barriers and safe access may add labor or reduce daily production.
Three Sample Quotes for Different Addition Scopes
Comparing complete scopes is more useful than comparing headline totals or per-square-foot rates alone. The following examples represent plausible bids in different markets, not guaranteed prices; each assumes permitted construction and standard-grade finishes.
| Example | Scope | Labor and rate | Estimated total |
|---|---|---|---|
| Basic family room | 300 sq ft, slab, two windows, no plumbing | About 450 hours at $60-$85 per hour; $175-$250 per sq ft | $52,500-$75,000 |
| Bedroom and bath | 450 sq ft, crawl space, one bathroom, standard fixtures | About 700 hours at $65-$95 per hour; $240-$330 per sq ft | $108,000-$148,500 |
| Kitchen expansion | 600 sq ft, roof alteration, cabinets, plumbing, upgraded circuits | About 1,000 hours at $75-$110 per hour; $300-$400 per sq ft | $180,000-$240,000 |
The labor-hour figures indicate combined trade effort, not the number of hours billed by one worker. The total may include materials, contractor overhead, and installation, so multiplying the labor estimate and adding the full per-square-foot total would double-count labor.
For a fair comparison, request the same room area, finish level, window count, HVAC approach, and allowance amounts from each bidder. Ask whether flooring, painting, insulation, permits, disposal, and utility connections are included. A low bid with exclusions can cost more than a higher quote with a complete scope.
Which Scope Changes Can Cut a Single-Storey Extension Price?
Reducing complexity usually saves more than choosing the cheapest finish available. Keeping the addition rectangular, placing it close to existing plumbing, and using a simple roof connection can reduce design, framing, and labor costs. Eliminating even one unnecessary bathroom can save $15,000-$40,000, depending on the drain route, fixture quality, and local rates.
Compare standard windows, siding, flooring, and cabinets with premium selections before finalizing plans. Choosing stock rather than custom cabinets can save several thousand dollars; limiting large window openings may avoid structural beam costs. Avoid changing room dimensions after permitting, since redesign, material restocking, and schedule disruption can each add expense.
Do useful preparation only when the contractor confirms it will not interfere with warranties or inspection requirements. Clearing furniture, confirming access, and documenting existing utility locations can reduce delays. Homeowners should not handle structural demolition, electrical work, or plumbing without proper qualifications and approvals.
Get at least three itemized bids and compare allowances, exclusions, payment schedules, and change-order rates. Ask contractors to price optional upgrades separately and identify a fixed scope for the base quote. A contingency of 10%-15% helps cover hidden conditions without building every possible risk into the initial contract.