Building an extension: where to start.

Thinking about building an extension? Not sure where to start?

The film further down this page covers four major things to think about before you decide whether a house extension is the right option for you. Every photograph here is a frame from it.

An extension, or a loft or garage conversion, creates space without the stress of moving, which matters if you like where you live. But first you need to find out whether an extension is even possible on your plot.

That is what this page is about. Not what it will look like. Whether it can be built at all.

Maybe you want an open plan kitchen diner. Perhaps an office. If you have a growing family, a nursery or playroom. If you train at home, somewhere to do it.

Whatever the reason, planning the extension properly is the enjoyable part. When you describe the space you want and a builder or architect puts it on paper, that is a good day.

But the drawings can only come after the fundamentals. Here are the four that decide whether there is anything to draw.

If you're building an extension over an existing drain you need a build over agreement from your local water authority. It gives assurance that you have given the sewer the correct clearance from your new foundations.

This is the one that catches people latest and hurts most, because it is invisible until someone lifts a cover. Find the runs before you design around them, not after.

If cables need to be moved or extended, the electricity board may have to do that work, and they work to their timescales rather than yours.

You also need to know whether your consumer unit can take the additional circuits the extension needs. A board that is already full is not a small line item once you are on site.

A digger needs to get to the rear of the property to dig out the footings. You do not want to be moving that much earth by hand, and pricing it that way will change what the job costs.

Mini diggers are normally upwards of 690mm wide. Measure the tightest point of your access, usually a side gate or the pinch between the house and the boundary, before you assume machinery can get round.

Building an extension increases the downward force on the land, so you need to be confident the ground can take the additional weight.

You may also need to consider next door's situation, as John does in the film. What is happening on the other side of the boundary, and what has already been built there, is part of your ground conditions whether you like it or not.

If an extension is the right option, there are a few more things to check.

Since legislation introduced in May 2019, some home extensions can be carried out under permitted development. Depending on the size, you may still need planning permission, and the loft conversion guide covers the permitted development volume allowances in detail because the same rules sit behind both.

The honest order is: establish it can be built, establish what consent it needs, then design it. Doing those in the other order is how people end up with drawings they cannot use.