Side extensions

In theory, yes. But it all depends on the space you have available next to your semi-detached house. If you follow certain rules, you can build a single storey side extension using permitted development rights, which means you don’t have to get planning permission. But if you’re wanting to build a two storey side extension, you’d need planning permission.

The cost of a side extension depends on a range of factors, including the size of the project and the materials and finishes you choose.

According to averages across recent Resi projects, a single storey side extension usually falls between £46,000 and £75,000, rising to £56,000 to £90,000 in London and nearby areas. A double storey side extension costs more, averaging £94,000 to £157,000, or £113,000 to £190,000 in and around London.

These figures only cover construction, so you’ll also need to factor in decoration, fixtures, fittings, and landscaping. If you want advice on financing your side extension, the Resi Finance team can help you explore your options.

Adding a side extension is a big decision, so it’s worth weighing up the pros and cons carefully. Consider the temporary disruption to your home and the project cost, which can be quite high considering the increase in labour and material prices over the past few years. Then balance that against the long-term benefits. A side extension can give you more space to cook, work, relax, or entertain, while also hopefully increasing your property’s value.

Looking at both practical and lifestyle advantages will help you decide if a side extension is the right choice for you.

Any extension that impacts the boundary line or existing party wall will need a party wall agreement. That means you’ll have to inform your neighbours of the work at least two months before it starts. If they give written consent to your project, you don’t need to do anything further. But if they don’t reply within 14 days, or if they write back dissenting, you’ll need to have a party wall agreement drawn up. This needs to be done by a party wall surveyor, which the Resi team can help you find.

The boundary is the point that separates your house from your neighbour’s, and with a side extension you can build right up to it, or even astride it (half on your land, half on your neighbour’s land). This would require a party wall agreement.

Alongside wanting more living space, adding value is one of the main reasons people choose to extend their homes. And while you can never guarantee how much value you’ll add, you can be relatively confident that increasing the footprint will increase your your house price.

According to Resi’s data, a side extension could add an average of 3% additional value, and even more for a double storey. But remember that every home is different, and your location, the size of your extension and various other factors will all affect how much added value you get. For a better idea of how much you could drive up your property’s worth, try Resi’s free extension valuation calculator.

Every project is different, so your timescale might be different to other side extension projects. As a rough guide, a house extension in the UK usually takes between 7 and 12 months from start to finish. That includes working with an architect on design, the planning permission process, getting detailed technical drawings, choosing your contractors, and then the build.