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How Big Can You Build House Extension?

Updated: Aug 2, 2023

Renovating and extending a home is many people’s dream for life. And if that sounds like you, you probably want to build the biggest and the largest house extension to maximise the potential of exciting interior spaces.

But, how big can you build?

How far can you extend?

The good news is that you can build a very large house extension under permitted development rights using the prior approval larger home extension mechanism.

Thanks to the permitted development rights for larger extensions, you can build a house extension up to eight metres beyond the rear wall of your detached house or six metres out from your semi-detached or terraced house.

We will explain the prior approval procedure in detail and show you how to extend your terraced or semi-detached house over three and up to six metres, and your detached house over four and up to eight metres.

You will learn the fundamentals of prior approval applications for larger home extensions, specific conditions you need to comply with for prior approval house extension and how to get your consent with ease using the neighbour consultation scheme.

So let’s begin...

What is the Prior Approval Mechanism for Larger Home Extensions?

Under the conditions set by under Part 1 Class A of the permitted development rights, you can build a large single-storey rear extension subject to meeting the requirements of the Town and Country Planning (General Permitted Development) (England) Order 2015 without seeking planning permission.

This is also known as prior approval larger home extension mechanism or the neighbour consultation scheme.

How Big Can I Build Using a Prior Approval Application for Larger Home Extension?

If you live in a terraced or semi-detached property, permitted development rights allow you to extend the rear of your house by six metres.

If you live in a detached property, the allowance is eight metres.

Yes, you read that right!

But beware: you will need to take the measurement of these projections from the original rear wall of your property and meet the certain conditions explained below.

Which Properties Are Excluded from the Neighbour Consultation Scheme?

According to the prior approval legislation for larger house extensions, some properties are excluded from the scheme. These include:

  1. Properties that are flats or maisonettes.

  2. Properties that are statutorily listed buildings.

  3. Properties that are within a conservation area, Area of Outstanding Natural Beauty, National Park, World Heritage Site or a Site of Special Scientific Interest.

  4. Properties that have had permitted development rights removed by condition or through what is known as an Article 4 Direction.

If your property type is listed above, it means that you don’t benefit from the prior approval for the larger home extension scheme. But don’t worry. There’s a solution.

Let us be clear: you can still build a very large house extension by submitting a householder planning permission with the help of professional extension designers and achieve the same or better results.

What Are the Conditions I Need to Comply with to Build My Large House Extension?

In accordance with The Town and Country Planning (General Permitted Development) (England) Order 2015 and the prior approval larger home extension procedure, you can extend your terraced or semi-detached house over three and up to six metres, and your detached house over four and up to eight metres.

However, you will still need to meet certain conditions of Part 1 Class A. These include:

  1. Your proposed extension should be single storey.

  2. If the extension at any goes beyond the line of the side of your house, overall it can’t be wider than half the width of the existing house.

  3. Your proposed extension should not exceed 4 metres in height, as measured from the highest point of natural surface ground.

  4. If your extension is within two metres of the boundary of the house, the eaves of your proposed extension should not be more than three metres in height.

  5. By building your proposed extension, no more than half the area of land around the original house (as it stood on 1 July 1948) should be covered by buildings.

  6. The materials of your proposed extension should match the existing appearance of your house.

Let’s not forget: failure to comply with these conditions will result in your application being rejected. Make no mistake about it!

How to Apply for Prior Approval to Build a Larger Home Extension

With prior approval applications for larger home extensions, you can build a house extension between 4 and 8m in length beyond the original rear wall for a detached house and between 3 and 6m in length beyond the original rear wall for all other houses.

However, if you wish to build a large house extension under the prior approval neighbour consultation scheme, you first need to apply to your council for prior approval, which is different from a planning application.

So what does all this mean?

Quite simply, this type of application allows your council to consider whether your proposed larger home extension qualifies for the conditions set out in the prior approval legislation and the likely impact of your extension on your neighbours and their homes.

When submitting your prior approval application for a large house extension, you should provide the following information with your completed application form:

  • A written description of your proposed large house extension including how far the enlarged part of the house extends beyond the rear wall of the original house; the maximum height of the enlarged part of the house; and the height of the eaves of the enlarged part of the house.

Don’t forget: the height of the eaves is measured from the natural ground level at the base of the external wall of the extension to the point where the external wall would meet (if projected upwards) the upper surface of the roof slope. Parapet walls and overhanging parts of eaves should not be included in any calculation of eaves height

  • Planning drawings, ideally with existing and proposed floor plans and elevations indicating the site and showing your proposed large house extension with measurements

  • The addresses of any adjoining premises

  • Your contact details

A word of caution: you should not start the construction of a larger rear extension without successfully completing the notification and prior approval process. Because the prior approval mechanism cannot be done after the extension is built, you would need to submit a retrospective full planning application to get it authorised, and that’s not a process we would recommend.

What Happens When You Submit a Prior Approval Application?

Once your council has received your prior approval application, they will consult your adjoining neighbours by letter, with a minimum consultation period of 21 days. The prior approval larger home extension legislation sets out specific requirements as to the information that must be provided by the consultation letter, including a description of the length, maximum height and the eaves height of the extension.

Here’s the most important part: if your council does not receive any comments within 21 days and decides that your proposal meets all of the conditions required for building a larger house extension under the conditions set under Part 1 Class A of the permitted development rights, your proposal will be considered permitted development. This means that your larger house extension does not need further planning consent.

But here’s something really interesting: if your council doesn’t decide on your application within the application process – which should take no longer than 42 days from the date the application is valid – you will have the right to submit an appeal to the Planning Inspectorate for non-determination.

What Happens If My Neighbours Object to My Prior Approval Application?

If any of the owners or occupiers of the adjoining premises object to your proposed development, then your council must assess the impact of your extension on the “amenity” of all adjoining premises, not just the adjoining premises that objected to your development. “Amenity” is a frustratingly vague piece of planning jargon – in this case, it broadly means quality of life. (Although, importantly, disruption and noise caused by construction are not part of what the council can consider).

Perhaps unsurprisingly, during this stage, the council may require you to submit further information to justify how you address the concerns relating to that objection. At this stage, you should submit sufficient factual evidence to prove that your extension will not have a detrimental impact on neighbouring properties.

Your council will assess your application for prior approval against all of the limitations and conditions set for prior approval applications for larger home extensions and take into account any objections received. Following their assessment, your council will issue a decision indicating whether your large household extension would comply with condition Part 1 Class A of the relevant legislation.

The good news is, if they don’t see any reason to refuse your application, then they will grant prior approval, most probably subject to conditions.

Can My Council Refuse My Large Home Extension Application?

At first glance, prior approval legislation for large house extensions may seem straightforward. However, getting approval for a planning application including for prior approval application is quite a skill to master; one which can sometimes take years to perfect

Furthermore, it is not just whether your larger house extension meets the requirements of Part 1 Class A that affect how the decision is made. Other factors, such as not submitting all sufficient information regarding the proposed development that your council may reasonably require can end up with your application getting refused. And, as we’ve said, the council has to be satisfied it won’t make your neighbours’ properties less enjoyable or convenient to live in.

If the council refuses your prior approval application, then this indicates that the works would not comply with Part 1 Class A. They will also indicate whether or not the works would comply with any of the other limitations or conditions within Part 1 Class A.

As an applicant, you will have the right to submit an appeal to the Planning Inspectorate against the refusal.

Can I Start Building My Extension Straightaway?

The prior approval larger home extension legislation states that you should not start building your large house extension without receiving a written notice from your council that your prior approval is not required.

With receiving your “prior approval granted” or “prior approval not required” decision, you can move onto the next stage: Building Control.

As with all house extensions, you will need to submit a building regulations application to seek Building Control approval before commencing the construction work. Your architects and builders should ensure that your home extension will be designed and executed in accordance with the building regulations.

And one last thing: you will need to notify your council as soon as reasonably practicable after the completion of works.

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