How long does a house extension take to build?

It’s one of the first questions every homeowner asks, and it’s a fair one. You’re about to live through months of dust, vans on the drive and decisions about things you’d never thought about before. Knowing roughly how long it all takes helps you plan your life around it, and it’s the difference between a project that feels controlled and one that feels endless.

Here’s an honest answer, broken down into the bits that actually eat the time.

The short answer

For most homes in Bristol, the full journey from “let’s do this” to “we’ve got our space back” runs somewhere between 6 and 12 months. The build itself is the smaller part of that.

Single-storey rear extension: roughly 10 to 16 weeks on site once work starts.
Double-storey extension: roughly 14 to 22 weeks on site.
Side return or wrap-around (common on Victorian terraces in Southville and Bishopston): usually 12 to 18 weeks, partly because of the structural work involved.

These are working-week figures for a properly resourced job. The reason the overall journey is longer is everything that happens before a single brick is laid.

The three phases (and where the time really goes)

1. Design and drawings

This is where most people underestimate the calendar. Getting a measured survey, agreeing a design with an architect or designer, and producing the drawings typically takes 4 to 10 weeks, depending on how decisive you are and how busy your designer is. The more you change your mind here, the longer it runs, and that’s no bad thing. Changing a wall on paper costs nothing. Changing it once the blockwork is up costs real money and real time.

2. Approvals

There are two separate things people muddle together:

Planning. Many single-storey extensions fall under Permitted Development (rights that let you build within set limits without full planning permission). If yours does, a Lawful Development Certificate is still worth getting and takes around 8 weeks through Bristol City Council. If you need full planning permission, budget 8 to 12 weeks, sometimes longer if neighbours object or the case officer asks for changes.
Building control. This is the separate sign-off that your build is structurally sound and safe. It runs alongside the work, with an inspector visiting at key stages (foundations, drains, and so on).

If your home shares a wall with a neighbour, the Party Wall Act comes in. You must serve notice, and neighbours have time to respond. Done early it costs you nothing on the calendar. Left late, it can stall a start date by a month or more.

3. The build

This is the part you picture: foundations, walls, roof, windows, then the slower interior fit-out. A rough order of events for a single-storey rear extension:

Set up, dig and pour foundations (1–2 weeks)
Brick and blockwork up to roof (2–4 weeks)
Roof structure and weatherproofing (1–2 weeks)
Windows, doors and making it watertight (1 week)
First-fix electrics and plumbing, plastering (2–3 weeks)
Second fix, flooring, decorating, snagging (3–5 weeks)

The back half always feels slower than the front. Walls go up satisfyingly fast; kitchens, tiling and the hundred small finishing jobs do not.

What actually causes delays

In fifteen years on Bristol sites, the same culprits come up again and again:

Weather. Foundations and groundwork hate heavy rain. A wet fortnight in a Bristol winter can genuinely push you back a week or two.
Mid-build changes. “Could we just move the door?” is the single biggest avoidable delay. Every change ripples through electrics, plastering and ordering.
Materials and lead times. Steel beams, certain bricks, and made-to-order windows can have 6 to 10 week lead times. A good builder orders these early; ask when yours plans to.
Surprises in the ground or the existing house. Old drains, dodgy foundations, or non-standard Victorian construction. Honest builders flag the risk of these upfront rather than pretending they can’t happen.
Cashflow and decisions. If choices on tiles, taps and paint aren’t made in time, trades stand idle. Decide early.

Get a written schedule before you start

This is the most important practical tip in the whole guide. Before any deposit changes hands, ask your builder for a written programme that shows start date, the main stages, and a realistic finish. It doesn’t need to be a fancy chart, just honest dates on paper.

A written schedule does three things: it tells you whether the builder has actually thought the job through, it gives you something to talk about when reality drifts (and it always drifts a little), and it stops the vague “should be done by summer” promises that cause so much friction. Pair it with a fixed written quote so you know the money and the time in the same breath. If a builder won’t put either in writing, that tells you something.

If you’re weighing up an extension and want a straight answer on how long yours would realistically take, we’re happy to come and see it. Get a free quote with no obligation, a clear written schedule, and a fixed price, or take a look at our recent work and the areas we cover first.