Additions add new footprint, which means new foundations, structure and a roof that must tie into the existing house. That makes them among the largest and most variable residential permits.
What the permit filings show
Based on 6,601 residential building-permit filings. The median home addition / extension valuation runs from $75,000 in Austin to $166,600 in New York, about 2.2× the cheapest city. Within a city the spread matters too: the middle half of San Francisco filings spans 447% of its median, versus 133% in Los Angeles.
CityTypical range (P25–P75)MedianFilings
Austin$25,000 – $170,000$75,000755
Los Angeles$40,000 – $140,000$75,0002,337
San Francisco$15,000 – $350,000$75,000242
Chicago$80,000 – $300,000$150,000494
Seattle$72,500 – $275,000$150,0002,498
New York$45,750 – $426,760$166,600275
Filings are declared valuations from each city's open permit dataset, limited to residential records classified as home addition / extension work and valued between $1,000 and $2,000,000. A city is listed only when at least 30 filings qualify. Full methodology. Data refreshed September 2026.
What the numbers say
Sample size varies by city too — 2,498 qualifying home addition / extension filings back the Seattle figures above, versus just 242 for San Francisco — and while both clear the site's 30-filing minimum, Seattle's far larger sample settles into a steadier shape than San Francisco's, which sits closer to that threshold.
Zoom out across every city on this page and the legitimate range is wider still: the lowest 25th-percentile figure is $15,000, in San Francisco, and the highest 75th-percentile figure is $426,760, in New York — a 28.5× gap that reflects different cities, codes and building stock, not overcharging.
In Los Angeles, quotes cluster tightly: the middle half of filings sits between $40,000 and $140,000, so a quote near the $75,000 median there is the safer assumption. In San Francisco, that band runs from $15,000 to $350,000 — wide enough that a quote toward either edge is still plausible, so treat San Francisco's median as a starting point for a conversation, not a target.
Why additions vary so widely
A single-room bump-out and a two-story extension are both additions. Floor area, foundation type, number of stories and the level of interior finish all scale the cost, and the range published here reflects that mix of projects.
Beyond construction
Design, structural engineering, permits and utility connections can be a substantial share of the total and are sometimes excluded from a declared valuation. Site drainage and connecting to existing services often add cost that is not obvious at the start.
Managing the project
Get a fixed scope and finish schedule in writing, confirm who handles engineering and inspections, and agree how changes after approval will be priced. Late design changes are a common source of budget growth.
What drives the cost of an addition
Floor area is the biggest single lever — cost scales with square footage roughly in line with the finish level chosen, so a modest bump-out and a full second-story addition are really two different projects wearing the same permit label.
Foundation type follows from the site: a slab addition on level ground is simpler than one requiring a crawlspace, basement or deep footings on a sloped lot — one of the least visible but most consequential costs in the whole project.
Tying the new roof, structure and exterior into the existing house is a cost with no equivalent in new construction — matching rooflines, siding and window styles, and working around an occupied home, all add labor. Interior finish level then scales the top of the range independently of the structural shell.
A worked example
Take Seattle, the city with the most filings behind its figures (2,498). Suppose two contractors quote for the same home addition / extension: one at about $150,000, in line with the local median, and another at about $330,000, well above the local P75 of $275,000.
Neither figure alone says which contractor is right: $150,000 sits where most Seattle filings land, a reasonable opening point, while $330,000 sits above where three-quarters of them do — not necessarily a red flag, since a second story, a challenging foundation or a higher interior finish level can all justify it — but a scope worth having spelled out before you sign. Both figures are illustrative.
What an addition permit valuation leaves out
The filed valuation covers the construction scope declared for the addition: foundation, structure, envelope, services and finishes for the new space. It is a declared figure, not a reconciled invoice.
Design, structural engineering, permitting and utility connection fees can be a substantial share of an addition's true cost, and are sometimes bundled into the declared valuation and sometimes billed and paid separately from the construction contract.
Site drainage changes, temporary accommodation if the work displaces part of the household, and redesign after initial approval are the kinds of costs that surface as the project proceeds and rarely appear in the original filing.
Use your quote to test the range
What this project can cover
An addition or extension can include foundations, structure, roof tie-in, envelope, services, and interior finishes for the new space.
What may sit outside it
Land purchase, temporary accommodation, major utility upgrades, furnishings, and redesign after approval are commonly outside the filing value.
Checks before you accept a quote
Compare floor area, foundation type, and finish schedule.
Confirm planning, structural design, and inspection responsibilities.
Price utility connections and site drainage separately.
Questions to ask before you sign
What foundation type is assumed, and how would the price change if site conditions require something different?
Is the new roofline, siding and window style being matched to the existing house, and is that reflected in the quote?
What finish level — flooring, cabinetry, fixtures — is the quote based on, room by room?
Who is responsible for structural engineering, permitting and utility connection costs?
How will drainage and grading around the new addition be handled?
How are changes requested after the permit is approved priced?
What is the payment schedule, and is it tied to completed milestones rather than calendar dates?
Questions people ask
Why do additions with the same square footage cost such different amounts?
Foundation type, how much the new structure has to tie into the existing house, and interior finish level all vary independently of floor area, and any one of them can move the price substantially for the same size addition.
Is a second-story addition the same project type as a ground-level bump-out?
They're both filed as additions here, but a second-story addition typically needs more structural work — reinforcing what's below it, tying into an existing roof structure — so it tends to sit above the median rather than at it.
Why might the typical cost be similar across cities but the top of the range very different?
A similar median with a very different P75 usually means most projects are a similar size and scope across those cities, but one has a longer tail of larger or higher-specification additions pulling its upper range higher.