1. How to decide what to use an extension for and where to build it?

Your extension is as individual as your home, so before you think about costs or contractors you need to have a firm idea about what purpose your extension will serve, and where it will be built.

The design and costs of a wrap around extension vs. a two-storey rear extension will vary significantly, so think about what space you need, what it will be used for, and where on your property will be the best place to build.

It is always worth considering natural light and where your property benefits the most from sunlight, and how much outdoor space you are willing to sacrifice to create the foundations for your extension.

2. How do I start planning an extension budget?

The first consideration is your budget. You might be looking to remortgage to cover the costs, be using savings, or plan to take out lending for the cost of building your extension.

This in itself requires some thought and planning to ensure that you are getting the best deal, and are not losing out on interest payments.

If you are working with a designer or architect, be open about your maximum budget. This will help them to recommend the best design, layout and materials to achieve your requirements whilst remaining on budget.

3. Will I need Planning Permission for an extension?

Once you know what sort of extension you wish to build and how much it will cost, the next step is to consider whether you need planning permission. The cost of an extension application is £206 but the process can take several months should design amendments be required, so it is sensible to put in your application in good time.

Some extensions can be built under permitted development (PD) so will not require planning permission. However, you will need to think about Party Wall Agreements, Building Regulations and any permission required should your property be in a Conservation Area or be a Listed Building.


4. Are any works required for site access to build my extension?

Whilst you are waiting for planning permission, think about access to your site. Particularly where foundations are being built, your contractor will need to be able to use equipment on site that can be difficult in densely built urban areas.

If your foundations need to be dug by hand this will add extra time to your extension build, and should your contractor need to use specialist equipment due to site restrictions this could add to your budget.

5. What drainage works will be required before my extension is built?

Where foundations are being dug for an extension, this often impacts existing drainage pipes.

You may need to have drainage pipes moved before building work begins, and might need to contact your local water company to obtain their permission to move public drainage services.

6. Do I need a Party Wall Agreement for my extension?

A party wall is a boundary between your property and your neighbour over which you have joint ownership. If your extension builds up to, or very close to, a party wall, you will need a formal agreement between yourself and your neighbour.

Obtaining such an agreement can be a simple process and require a Party Wall Surveyor to draw up an agreement at a cost of around £200. However, should the negotiations be complex or your neighbour be unhappy with your extension plans this can take considerably longer and cost much more.

It is always wise to have a chat with your neighbours before any works progress to ensure that you are in a good position to draw up an agreement.

7. Do I need insurance for my extension?

Usually, your home insurance will not cover any costs incurred by damage that happens whilst building work is happening on site. It is important to be properly insured since should anything happen, you could be looking at heavy costs to put things right.

Look around at site insurance policies and costs – your insurer may offer an extension for the duration of the work, or you may need to take out specific insurance for your build.

Frequently Asked Questions

How long does an extension take to build?

Typically, a simple extension will take around three months in build time, and another three to six months in the planning, design and obtaining permissions. Larger and complex builds will take around six months to construct.

In total it is best to expect the total project to take around one year, and ensure that you have contingencies built in should any delays occur or any applications be rejected.

Do I need a Lawful Development Certificate?

A Lawful Development Certificate is not mandatory but is worth having particularly if your extension did not require planning permission.

This certificate is issued by your local authority and verifies that your extension was built under permitted development criteria, did not need planning permission, and has been completed lawfully.

The application cost is half of a planning permission application at £106, and is useful to have should you have any future queries or come to sell your home.

Does my extension need planning permission?

Some extensions can be built under permitted development (PD) and therefore do not require planning permission. These criteria include conditions such as:

  • Rear extensions may be no more than 8 metres for a detached property with a single-story extension or 3 metres for a double-height build
  • Single-storey extensions may be no higher than 4 metres and must not exceed the height of the original property
  • Double-height extensions must not be within 7 metres of the rear boundary
Mike Alexander
Hey there, I'm Mike - writer and part time home improvement expert at Refurbb. Since owning and refurbishing my own property in 2018, I've since been developing rental properties, writing about my home improvement endeavours, sharing what I've learned and connecting readers to reputable tradespeople in the UK.

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