How to plan an extension

Building a new house extension, whatever the size, is a big task that requires lots of time and patience. 

From how to start planning an extension to who will design and build it, and avoiding feuds with neighbours, here are some key questions to consider before embarking on a new extension.

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How do I start planning an extension?

From planning permission and party wall agreements to finishing touches, tackling an extension can be overwhelming. Break it down into digestible chunks and begin with thinking about the basics. We've included some initial questions below to get you started.

Ideally, you'd like to end up with a brand new space that not only looks great and provides more room, but adds value to your property, too. 

But first, consider these questions:

Is extending your property right for you?

Can’t bear the thought of moving house but current home feeling on the small side? Perhaps it's the upheaval and cost – or it's simply leaving behind a home you love. Whatever the reason, extending could be the ideal solution and it means you can tailor the space to suit your living situation.

If you've been thinking about extending your home, consider whether your plans are going to add value and how much extra space the new extension will bring. 

Depending on the quality of the extension and the location of your property, the increase in value might be substantially more than the initial investment of the extension. But if it's not, is the extra space worth the money? How does it compare with the cost of moving house and the upgrade to a bigger property? 

Talk to local estate agents to get a good idea about your local market so you can begin your extension with confidence. 

For further insights, read Extension costs: how much should you expect to pay in 2024? and Cost of moving house now more than £14,400: how to save money

Who will design and build your extension?

One of the first things you’ll need to think about is who is going to design your new extension. Choosing the right person to draw up your plans is as important as choosing the right builder.

You can design it yourself, opt for a build and design company or work with an architect or architectural technologist.

The key is to do your research, look at previous work and ideally speak to past clients. Choose someone who most closely aligns with your design aspirations – and who you feel you can work with best. The same goes for picking a builder. 

Read advice from an architect and builder for a detailed Q&A and their top tips. 

You can also head to Which? Trusted Traders to find a reliable, local tradesperson:

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Is the property leasehold or freehold?

If you own the leasehold rather than the freehold to your property (often true of flats) you’ll need to check the details of your lease to ensure you have the rights to alter the property. You might need to get the approval from the freeholder, which might mean additional plans are required to get sign off. 

How to keep the peace with neighbours?

Everyone wants to avoid feuds with their neighbours, but building projects can be a major cause of disputes. Try and have an honest and open discussion in person beforehand to hopefully avoid problems down the line. 

Let them know about your plans well in advance, particularly if the works may cause disruption. If planning permission is required, your neighbours will be consulted by the local planning authority by post.

It may also be that you need a party wall agreement (a party wall is a shared wall, usually between a terrace or semi-detached house, which divides the homes of two separate owners). Party wall agreements are most commonly needed for loft conversions and extensions that require the insertion of steel supports, a damp proof course or new foundations.

Should you move out when building an extension?

The upside to moving out is avoiding disruption, dust and noise. Living in a dusty building site is not easy and, depending on what extension you go for, you could be without a kitchen or bathroom for a while. Your property could even be missing a rear wall or roof, heating and running water, which is particularly miserable in the winter.

Living on-site means you'll be able to oversee building work, make quick decisions and keep things moving. You'll also save a lot of money, and finding short-term lets can be tricky. 

What extension should you go for?

You probably have a rough idea of what extension you'd like – depending on whether you live in a flat or house, if you want to build out as much as you can or simply convert your garage.

If you're deliberating between one or two storeys, be aware that a two-storey extension won’t cost much more per square metre – you’ll need to pay for the most expensive elements however many storeys you choose. Head to our extension costs page to see the difference.

When deciding, think about what access you have. For example, off-street parking or a side entrance for bringing materials to the plot and accessing the site.

Head to our guide on house extension and conversion ideas to compare the different ways to extend or convert your home.

We also have dedicated guides to loft conversions and conservatories, covering costs, insider tips from homeowners, jargon busters and style ideas.

Will you need planning permission?

This is a vital point to consider when planning an extension. 

Head to our page on building regulations and planning permission for more in-depth advice.

Should you get extension insurance?

Before building an extension, it’s important to contact your home insurance provider to let it know of your plans. The extension will likely increase the rebuild cost of your house, which is something insurers take into account when pricing premiums. What's more, building work could put the property at risk of damage. 

Call up your insurer and it will let you know if your current policy will cover the new extension. You may find your premiums increase – but if you don’t let your insurer know and there’s a problem with the property in the future, your policy could be void. If for some reason your provider is unable to cover the property now, you’ll need to find a new one before your cover is cancelled.

Your insurer also needs to know if you’re staying somewhere else during the works. If you’re having serious work done, consider specialist renovations insurance which covers against things going wrong with building work, materials and property owners’ liability.

Check that any builder working on your property has professional indemnity insurance to cover the costs should something go wrong.