How much does a home addition cost? (US, Canada & UK guide)

Adding space to your home is often cheaper and less stressful than moving. But “how much will it cost?” is the question almost every homeowner struggles with. Online calculators give a single number, builders give very different quotes, and the real cost only becomes clear once work has started.

This guide gives you realistic cost ranges for the most common projects in the United States, Canada and the United Kingdom, explains what drives the price, and shows you how to build a budget you can trust.

Typical cost ranges by project type

The figures below are indicative ranges for a standard finish level, including labour and materials but excluding land, design fees, permits and furniture. They vary a lot by region: expect the upper end of the range (or more) in expensive metro areas such as New York, San Francisco, Toronto, Vancouver or London.

Project	United States (USD per sq ft)	Canada (CAD per sq ft)	United Kingdom (GBP per m²)
Single-storey rear or side addition	200 – 400	250 – 450	2,000 – 3,000
Second-storey addition	250 – 500	300 – 550	2,200 – 3,500
Garage conversion	100 – 200	120 – 250	1,000 – 1,800
Loft / attic conversion	150 – 300	180 – 350	1,500 – 2,500
Detached ADU / garden annex	250 – 450	300 – 500	1,800 – 2,800

So a 400 sq ft (37 m²) single-storey addition in a mid-cost US region typically lands somewhere between $80,000 and $160,000, while a 20 m² single-storey rear extension in England outside London is often in the £40,000 – £60,000 range.

A quick note on units: 1 m² is about 10.8 sq ft. If you compare a UK article with a US one, always convert first.

Why the same addition can cost twice as much

Two additions of exactly the same size can have very different costs. The main drivers are:

1. Foundations and ground conditions

Soft or clay soils, sloping sites or a high water table can require deeper or reinforced foundations (the concrete base that carries the building). This is invisible once finished, but it can add thousands to the bill.

2. Structural work on the existing house

Opening a large wall between the old house and the new space usually needs a steel or engineered-wood beam and temporary supports. A second-storey addition may require strengthening the existing walls and foundations to carry the new weight.

3. Kitchens and bathrooms

Rooms with plumbing are the most expensive per square foot. An addition that contains a new kitchen or bathroom can cost 30–50% more than a simple living room or bedroom of the same size.

4. Finish level

Flooring, windows, cabinets, fixtures and lighting can vary enormously. Moving from “standard” to “premium” finishes often adds 25% or more to the total.

5. Location and labour market

Labour is usually the largest single cost. It varies strongly between regions, between cities and rural areas, and with how busy local contractors are.

6. Access and site logistics

Narrow access to the backyard, no space to store materials, or the need for a crane all increase costs.

The costs people forget

A realistic budget includes more than the construction itself:

• Design and engineering fees: architect or designer, structural engineer, and in some areas energy calculations.
• Permits and approvals: building permit fees in the US and Canada; planning and building regulations fees in the UK.
• Utility connections and upgrades: a larger electrical panel, extended heating or a new water line.
• Surveys: a land survey, soil report or, in the UK, a party wall agreement if you build near a shared wall.
• Contingency: we recommend keeping 10–15% of the construction budget aside for surprises, especially when working with an older house.

How to build a budget you can trust

1. Define the project clearly. Size, number of floors, which rooms, and the finish level you want. Even a rough sketch helps.
2. Get a first range. Use our free construction cost calculator to see whether your idea fits your budget.
3. Get an independent estimate. Before asking contractors for quotes, have an engineer estimate your actual project, stage by stage. You will know what a fair price looks like.
4. Compare quotes against your estimate. Differences become easy to spot: missing items, vague allowances or unusually high prices. Our guide on how to check if a builder’s quote is fair explains what to look for.
5. Keep your contingency. Don’t spend it on upgrades before the work is finished.

ADUs: a special case

In many US states and Canadian provinces, rules now make it easier to build an accessory dwelling unit (ADU), a small independent home on the same lot. Because an ADU is a complete dwelling with its own kitchen, bathroom and utility connections, its cost per square foot is often higher than a simple addition, even though it is smaller. Prefabricated ADUs can save time, but site work, foundations and connections are usually extra.

The bottom line

A home addition is a big investment, and the biggest risk is starting with the wrong number. Treat online figures as a starting point, then get a detailed estimate based on your actual house and design before you sign anything.

If you want real numbers for your project, send us your plans or a sketch: an engineer will calculate your addition line by line, fully online.