Yes, you can build a house extension in winter, and plenty of extensions are started between November and February. Cold and wet weather slows some stages and stops work on the worst days, but with sensible planning and protection the finished extension is just as sound as one built in summer.
This post explains which stages of the build the weather affects, the temperature limits builders work to, how a winter project is kept moving, and why an autumn or winter start can suit you. It also covers what to expect if you are living at home while the work goes on.
Which stages of an extension the weather affects
An extension is built in a rough order: groundworks and foundations, then walls, then the roof, then the inside. The weather matters most at the start, before the building is "weathertight", which means the walls are up, the roof is on and the windows and doors are in.
- Digging foundations: heavy rain can flood trenches, and frozen ground can't be built on.
- Concrete for foundations and floors: it needs to stay above freezing while it hardens.
- Brick and blockwork: mortar, the mix that bonds the bricks and blocks together, can be damaged by frost before it has set.
- Roofing: strong winds make working at height unsafe, and an open roof lets rain into the new structure.
Once the extension is weathertight, most of the remaining work happens under cover. Insulation, electrics, plumbing, plastering, joinery and fitting out carry on whatever the weather is doing outside.
The temperature limits builders work to
Local Authority Building Control (LABC) publishes guidance on working in cold weather. It says masonry shouldn't be laid when the temperature falls below 2°C, and new walls should be covered with polythene or hessian if the forecast is for anything colder.
For concrete, the guidance is not to pour it onto frozen ground, because the ground can move as it thaws and damage the new concrete. Ready-mixed concrete shouldn't drop below 5°C, and it has to be kept from freezing while it cures, which is the slow process of hardening and gaining strength. Cold weather slows curing down.
You don't need to police any of this yourself. It's useful to know, though, because it explains why your builder might stop bricklaying on a frosty morning. A lost day like that is a sign the job is being done properly, not a sign of things going wrong.
How a winter build is kept on track
A winter extension takes a bit more planning than a summer one. The main ways a builder keeps it moving are:
- watching the forecast and planning foundations and brickwork around the milder, drier spells
- covering trenches and new concrete so they don't freeze overnight
- protecting fresh brickwork with covers until the mortar has set
- getting the roof on and the windows in as early as possible, with temporary sheeting over anything left open
- using wet or frosty days for work that can be done under cover
It's still wise to allow a little more time than you would for a summer build. Plaster and screed, the smooth sand and cement layer laid over a concrete floor, take longer to dry in cold, damp air. That can push back decorating and laying flooring, because both need a dry surface underneath.
Building control inspections still go ahead
Every extension needs building regulations approval, and a building control inspector checks the work at set stages, such as the foundation trenches before any concrete goes in. Those inspections carry on through the winter. If the weather moves a stage by a few days, the inspection simply moves with it.
We've explained each visit in our post on what happens at each building control inspection.
Why an autumn or winter start can suit you
There are some good reasons to have your extension built over the colder months.
You can be finished for spring and summer. Start in the new year and, depending on the size of the project, you could be using the new room, and getting your garden back, by the warmer months.
The paperwork can be done first. Drawings, structural calculations and building regulations approval all take time, and planning permission takes longer where it's needed. Getting these sorted over the summer and autumn means the build can start as soon as you're ready. Our guide to planning permission for a house extension explains when you need it.
There can be more choice of start dates. Many homeowners want building work to begin in spring, so asking about a winter start can sometimes open up dates that wouldn't otherwise be available.
Living at home during a winter extension
Most people stay at home while their extension is built, and winter doesn't change that. The point that matters most is when the new part is joined to the house. That usually means opening up an outside wall, and it's best done once the extension is weathertight, so your home stays sealed against the cold for as long as possible.
When the opening is made, temporary screens and dust sheets help keep the cold and the dust out of the rest of the house. If the work affects your kitchen, heating or hot water, it's worth talking early about when any supplies will be disconnected or moved, and planning a temporary kitchen if you need one. In winter, a few hours without heating matters more than it would in July.
How we run an extension in the colder months
JW Carpentry & Build provides you with paperwork at the start setting out the works and the costs, and we help with planning and building regulations from the beginning. During the build we keep you informed at each stage, so you know how things are going on the days you're at work or the weather has kept the team off the brickwork.
If you're thinking about an extension next year and wondering whether to start over the winter, JW Carpentry & Build can look at your plans and talk through a realistic start date and programme. You can find out more and book a visit through our home extensions service.
