Home Addition Cost Calculator

Most home additions run $100 to $300+ per square foot, so a 400 sq ft room addition typically lands around $40,000–$80,000 and a larger or second-story project can reach $150,000+. The biggest drivers are the addition type, the finish quality, the foundation, and the systems you extend (HVAC, electrical, plumbing). A garage conversion is the cheapest per foot because the shell already exists; a kitchen extension or second story is the most expensive. There's typically a project minimum around $20,000, since even a small addition carries fixed mobilization and permit costs.

A garage conversion, at roughly $50–$120/sq ft, because the foundation, walls, and roof already exist — you're mainly paying for insulation, drywall, an HVAC extension, flooring, and windows. A bump-out (extending an existing room a few feet) is the next most economical since it shares an existing wall and can often be cantilevered without a full foundation. The most expensive are kitchen extensions and second stories, which add heavy plumbing/appliance work or structural engineering. Reusing an existing structure is almost always cheaper than building new footprint from the ground up.

Almost always. Any work that expands your home's footprint or height is structural and requires a building permit and inspections — and usually architectural plans to apply. Skipping permits risks fines, forced demolition, and problems at resale, since unpermitted square footage often can't be counted or financed. Budget a few hundred to a few thousand dollars for permits depending on your municipality and project size. The calculator includes permit and architectural-plans add-ons so you can build both into your estimate.

A lot — it's a multiplier on the whole addition. Basic builder-grade (laminate, standard fixtures) runs about 25% below standard; standard mid-range is the baseline; premium (hardwood, tile, upgraded appliances) adds about 35%; and luxury (custom cabinetry, stone, smart systems) can nearly double the base. Because quality multiplies every square foot, it moves the total more than most people expect — a premium finish on a 400 sq ft addition can add tens of thousands over builder-grade.

It depends on your site and goals. A concrete slab is the most common and affordable, poured on grade with the cost built into the base rate. A crawl space adds about $15/sq ft for stem walls and a framed floor, and is used on uneven terrain or where you want access to run systems. A full basement adds about $30/sq ft for excavation, poured walls, waterproofing, and egress — the priciest, but it also adds usable square footage. The calculator treats foundation as a selection so the estimate reflects your choice.

Often yes, but it's among the most complex and costly additions ($150–$300+/sq ft). A structural engineer must confirm your existing foundation and walls can carry the added load; if they need reinforcing, costs climb. The roof has to come off during construction, so you'll likely need to move out temporarily, and weather protection adds cost. Second stories make sense when your lot is small and you can't build out — you gain space without giving up yard, but you pay for the engineering and the disruption.

Adding a full bathroom to an addition runs about $8,000 on top of the base, and a plumbing rough-in for a kitchen or wet area about $4,000 — both offered as add-ons here. The biggest variable is distance to existing plumbing: running new supply and drain lines more than 10–15 feet from the main stack adds cost. An addition placed directly above or beside existing plumbing is cheapest; a wet room far from the stack is the most. Kitchen extensions cost the most per foot largely because of this plumbing and appliance work.

Generally yes, though the return varies by type and neighborhood. Cost-versus-value studies typically show a mid-range addition recouping roughly 55–65% of its cost at resale, with adding a needed bedroom or bathroom (where comparable homes have more) yielding the highest return. A luxury addition in a modest neighborhood returns less — you can over-improve relative to the block. The best ROI usually comes from bringing your home in line with, not far beyond, the comparable homes around you.

For a full addition, hiring a licensed general contractor is strongly recommended. A GC sequences and coordinates the foundation crew, framers, roofers, plumbers, electricians, and finish carpenters, pulls the permits, and carries responsibility for code compliance. Acting as your own GC can save 15–25%, but it takes serious time, construction knowledge, and risk tolerance — and mistakes in sequencing or code are expensive to fix. Most homeowners come out ahead paying a GC for a project of this scale.

A standard room addition (200–400 sq ft) usually takes 3 to 6 months from permit approval to final inspection; larger projects like a second story or in-law suite run 6 to 12 months. And that's after the permit process, which alone can add 4 to 12 weeks depending on your building department's backlog — plus design time before that. Foundation choice, weather, structural complexity, and inspection scheduling all affect the timeline, so build in a buffer beyond the contractor's best-case estimate.