Building an Extension: Our Simple Guide

(Last Updated: 26/03/2024)
7 min read
Building an extension is an exciting prospect as you'll benefit from more space and hopefully, an increase in your property's value. The downsides are the time it takes to complete the extension, issues with the local planning officer and under budgeting. The best advice is to plan properly and to over budget on time and the costs of the extension - something always slips!

This article guides you through what you need to do when extending a single storey or double storey extension and this includes:

  • What types of building extensions are there?
  • Do you need planning permission?
  • Starting the build
  • Building regulation sign off
  • FAQS when extending your property

    What types of building extensions are there?
Before choosing on the type of extension you want you should have an answer to each of the following:

  • What you want to achieve? - is it to gain more space? increase the value of your property? make better use of the current floor space? Having a clear goal from the outset is essential to a successful build as it'll guide the decisions you make throughout.
  • What are you allowed to build? - Planning permission restrictions and local planning rules may stop you from being able to achieve your goals so you should consult with your local planning officer before you start any build.
  • What can you afford? - one of the greatest challenges is budgeting your planned extension. The costs can escalate as the build rarely goes to plan. You should plan you build with a best and worst case scenario with all of your suppliers.

Typical types of residential extensions

Here are a few of the typical types of extension to residential properties:

  • Single storey extension (including a wrap around)
  • Double story extension
  • Loft conversion

    Do you need planning permission?
You make a planning application online through the Planning Portal. Every local authority in England & Wales accepts online planning applications through this portal. You can either draw the plans yourself or you can get the help of a specialist; architect, RICS surveyor or a structural engineer. For some smaller builds, you may even get your builder to help you with your application. Regardless of who does the plan drawings, every application should include:

  • The proposed plans for the site;
  • Any supporting documents (including national & local level requirements);
  • Planning application form (if paper submission); and
  • Correct planning application fee.

What are the National Requirements?

  • The standard application form
  • Locational plan
  • Site or Block plan
  • Ownership certificate
  • Agricultural holdings certificate
  • Design and access statement

You can choose to provide a 'paper' planning application and you can download a printable form by clicking here.

How much does it cost to submit a planning application?

The majority of planning applications attract a fee. There are some exclusions such as for listed buildings and for demolition in a conservation area, where a fee is not required.

The planning application fee varies depending on the type of extension/works you are planning. You can see the costs here - Guide to the Fees for Planning Applications in England. Planning applications must be accompanied with the correct fee for the application - so it is payment upfront before a decision is provided.
If the local authority refuse the planning application then there is no refund of the fee.

Householder Applications
The planning application fee for an alterations/extensions to a single dwellinghouse, including works within boundary is £172 and the local planning office have 8 weeks to provide you a decision.

Can you extend without planning permission?

Applying for planning permission isn't always required so it is important to check to see if the work you are planning falls under what is called "Permitted Development Rights". To find out what is included within permitted development rights you'll need to delve into the The Town and Country Planning (General Permitted Development) (England) Order 2015.

There are different categories that relate to Permitted Development Rights so make sure to look for the type of work you are going to do. Each category heading is broken into different classes. As an example of this here is the first section 'Development within the curtilage of a dwellinghouse' (such as single storey or double storey extensions).

Classes in the 'Development within the curtilage of a dwellinghouse':

  • Class A – enlargement, improvement or other alteration of a dwellinghouse (this will include a single storey side and rear extensions)
  • Class B – additions etc to the roof of a dwellinghouse
  • Class C – other alterations to the roof of a dwellinghouse
  • Class D – porches
  • Class E – buildings etc incidental to the enjoyment of a dwellinghouse
  • Class F – hard surfaces incidental to the enjoyment of a dwellinghouse
  • Class G – chimneys, flues etc on a dwellinghouse
  • Class H – microwave antenna on a dwellinghouse

When are Permitted Development Rights not permitted?

Here are some of the key exclusions for Class A – enlargement, improvement or other alteration of a dwellinghouse:
  • as a result of the works, the total area of ground covered by buildings within the curtilage of the dwellinghouse (other than the original dwellinghouse) would exceed 50% of the total area of the curtilage (excluding the ground area of the original dwellinghouse);
  • the height of the part of the dwellinghouse enlarged, improved or altered would exceed the height of the highest part of the roof of the existing dwellinghouse;
  • the enlarged part of the dwellinghouse would have a single storey and extend beyond the rear wall of the original dwellinghouse by more than 4 metres in the case of a detached dwellinghouse, or 3 metres in the case of any other dwellinghouse, or exceed 4 metres in height (for any dwellinghouse)
  • it would consist of or include the construction or provision of a verandah, balcony or raised platform, installation, alteration or replacement of a chimney, flue or soil and vent pipe, or an alteration to any part of the roof of the dwellinghouse

    Start the build
With planning permission granted or a confirmation that the works fall under Permitted Development Rights, the next stage is to start the works. Most likely you'll have already

Detailed drawings

What if you don't complete the planned works?

After planning is granted, you can sell the property with the granted planning application to allow a future buyer to take on the project.

Building regulation sign off

The Building Regulations are designed to ensure new buildings works meet health, safety, welfare, convenience and sustainability standards. They relate to the specifics of how a building should be constructed unlike planning permission which is about the principle of whether development should go ahead or not.

Frequently Asked Questions

What are the costs to extend your property?

The costs to extend your home vary considerably and depend on the level of your involvement in the build. A more DIY approach can save money, but can delay the build if you aren't working on the project full time. You might also save money through not instructing architects/specialist surveyors, however this may limit the imagination of the build. Here are some of the costs to factor in when building an extension:

  • Planning permission fee - For alterations/extensions to a single dwellinghouse, including works within boundary the cost is £172.
  • Plan drawings - the cost for this depends on who you get to complete the plans. If you instruct an architect their costs can range from 6% to 11% of the total build cost, however a RICS surveyor or structural engineer can provide support at a fraction of an architects cost, but the plans may lack imagination of what could be achievable with the extension.
  • Labour and materials - an application for building control sign off varies depending on local authority but is around £640.
  • Building control sign off - an application for building control sign off varies depending on local authority but is around £640.

Can I extend a leasehold flat?

For virtually all types of flat you are unable to extend. Where there are only 2 flats, one above another, classically the case with maisonettes/house conversions, it might be possible for the ground floor flat to extend outwards and for the upper floor flat to extend upwards, however there are likely to be planning permission considerations, which might disallow such projects and in any case, the freeholder's permission would have to be sought - and may well be withheld.

How much can I extend my house without planning?


How to extend my house cheaply?


What are the top issues when extending your property?

Andrew Boast of Sam Conveyancing
Written by:
Andrew started his career in 2000 working within conveyancing solicitor firms and grew hands-on knowledge of a wide variety of conveyancing challenges and solutions. After helping in excess of 50,000 clients in his career, he uses all this experience within his article writing for SAM, mainstream media and his self published book How to Buy a House Without Killing Anyone.
Caragh Bailey, Digital Marketing Manager
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Caragh is an excellent writer in her own right as well as an accomplished copy editor for both fiction and non-fiction books, news articles and editorials. She has written extensively for SAM for a variety of conveyancing, survey and mortgage related articles.