Clients always think that they want to build an extension in spring, as it has the mildest weather, but I would look at seasonal records for the past few years as ‘spring’ these days doesn’t start on the 21st March.
You tend to find that what we classically think of as winter, i.e. November and December are relatively mild, and it only starts to get cold in January and February, so you should expect frosts January, February, March and even, according to the Prince Song, sometimes it snows in April! However, some months are wetter than others, which is harder to predict. But to get true spring weather and to avoid frosts you should build in the end of April, May, June, through to December.
Timing your Building Work with the Seasons
When it comes to the things that are going to be affected by frosts you should work in reverse order. For example, renders, which are usually the last things to go on your project, need to be frost free.
Otherwise, you may finish the project, without having rendered, just before the frost starts in December or January, so you’ll need to wait months before you can start rendering, so it’s not unusual for many projects to seem incomplete for ages when all they need is the last coat.
In general, however, you can’t predict when a project will start and finish, so you should ignore seasons and timescales and just get started; there’s no best time to build an extension.
Additional things to think about
You will need to think about the extent of the project, and where you are going to live during the build.
Can the extension be built before you knock through into the existing? If so, you can live in the existing house for longer.
Can you live without a kitchen? You could set up a temporary camp somewhere.
Can you live in a building site? If not, where will you stay and how long for?
At least the build will go quicker if you aren’t there as the builders will be able to fully access your house. I would suggest that you factor in the cost of staying elsewhere during your build, and the fact that you will be moving twice, and you should consider putting things into storage.
Anticipating the weather and building times can be very difficult, so you should just get stuck in, be flexible and open-minded, and have somewhere to live during the build.