How Much Does A Home Extension Cost?

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home extension

If you’re considering extending your home, either with a single-storey extension or double-storey extension, one of the most important questions initially, is how much does a home extension cost. If you have never extended before or done any major construction work, then chances are you won’t have a clue.

As well as the overall building costs to factor, you will also need to include architectural fee’s, application fees for planning and building control, decoration and furnishing.

In this blog we will discuss the cost of building an extension, what may impact the overall cost of an extension and if a home extension is right for you.

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Is a home extension right for you?

Extending your home is a huge decision and one that most people will only do once in their life, so it is important that you get it right. Before any architectural plans or building work can start you need to make sure a home extension is right for your home and is financially viable.

If this is your forever home, then building an extension is a no brainer compared to moving to a bigger property. You will save money on solicitor fee’s, stamp duty and the overall moving costs. However, if you do plan to move homes in the future and you still want to extend then you need to make sure extending your home will add more value than the extension costs.

How much does an extension cost to build?

There are many factors that will determine the cost of a home extension, these include the size and shape of the extension, the amount of glazing that will be used, if a new kitchen or bathroom is required, the quality of the build, the list goes on.

One of the big factors is location. It is generally known that services in London will be considerably higher than services in Grimsby for example.

As a rule, most extension projects will cost between £1350-£2250 /m2 of new internal space. To most, the estimate sounds rather vague — that’s because it is. That is all you can expect when you have no detail or idea of the size or shape of the extension, what materials you’re going to use, how it is going to be built, and who’s going to build it.

  Small (3mx5m) Medium (4mx6m) Large (6mx8m)
 Basic £12-£16k £19-£26k £38-£52k
 Standard £16-£21k £26-£34k £52-£67k
 Premium £21-£30k £34-£48k £67-£100k

One of the most important things to remember when planning your budget is to allow a percentage for a contingency to cover the costs of any unforeseen items that may crop up during the build.

What can affect the cost of my extension?

When designing your new home extension, your architect or architectural designer will design the extension with your requirements and budget in mind. For example, windows and doors can be pricey and easily increase the cost of even a modest extension. Triple glazing, larger windows or bi-folding doors are all luxury items – for smaller budgets your architect will be able to provide alternative solutions to keep costs down.

When you have your approvals in place and the building work starts on site, ground conditions are one of the first items that can have huge implications on the overall cost of your extension. Most footings are done by strip or slab footings. These methods are typical traditional construction methods and are usually the cheapest option.

However, if you don’t have the ideal ground conditions then this won’t be possible, and an alternative footing will be required. Alternative options could include a concrete ground bearing slab design, pile design or ring beam design. All of these will need to be designed by a structural engineer which will be an additional cost.

When increasing the size of your home, you may need to increase the size of your fuse board. Most people don’t consider this until the electrician is on-site so it’s always advisable to get this checked out before the builders start.

When checking this, the electrician will also check the wiring. If the wiring is old, then it may be worthwhile to rewire the whole house while adding the new electrics. The same can be said for the existing plumbing and heating.

How eco-friendly do you want your new extension to be?

Building an eco-friendly home extension in today’s climate can be a tricky balance of trying to do the right thing while keeping costs down. With most things, doing anything a little different or bespoke will increase costs.

Ways to make your new extension more economically friendly include:

  • Heat recovery systems
  • Solar panels
  • Under floor heating
  • Choosing sustainably sourced materials
  • Increased Insulation
  • Air Sourced Heat Pumps

Additional Costs

Architectural Fees – Most home extensions will require some form of planning approval or documentation and Building Regulation drawings and approval. These drawings are usually done by an architect or architectural designer.

Depending on the size of your extension you may or may not require Planning Permission. Some smaller extensions will be classed as permitted development. This means planning permission is not required.

However, it is always recommended to apply for a certificate of lawfulness to confirm this in writing. Along with the application form, you will also require detailed drawings of the existing home and the proposed development.

As of November 2021, the cost of a certificate of lawfulness application to the council is £103.00 and the cost of a householder planning application is £206.00.

Building Regulation drawings are a set of detailed drawings and construction details that are submitted to the local building control department and go into much more detail than the planning application drawings. The application fee for Building control varies from council to council and the overall size of the extension floor area.

Structural Engineer – A structural engineer will be required to provide calculations if your new extension affects the structure and stability of your property.

For example, if you wanted to create an open living space by taking down a load-bearing wall, you will need a structural engineer to calculate the changes to the structure, so your builder knows what works are required to maintain the stability of the building. Often this involves installing a steel beam or RSJ to support what’s above.

If structural calculations are required, then these will also be required to be submitted to Building Control for approval, prior to works being carried out. Quite often, your architect or architectural designer should help you find a suitable engineer for your project.

This article was written by Katie Medd. Katie works for, a residential architectural practice, offering local and online services. Katie enjoys blogging about the architectural side of home renovations.

Are you a thought leader or industry expert, like Katie, and have some insight to share? Don’t hesitate to get in touch today!

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photo of Millie Archer

Millie is a perfectionist with a passion for property and writing articles. You’ll find her researching the latest housing trends and the newest up and coming areas worth investing in. Read more about Millie here.

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About Millie Archer 142 Articles
Millie is a perfectionist with a passion for property and writing articles. You'll find her researching the latest housing trends and the newest up and coming areas worth investing in. Read more about Millie here.

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