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A guide to building a house extension

Published on 28 February 2024

Posted in Advice & Reviews

by MKM

8 min read

House or Home extensions are increasingly popular for a whole host of reasons. For many, the main reason is space; as families grow, space requirements change and often increase. Instead of exploring the process of moving, homeowners find it more useful to expand their existing living spaces.

This could take the form of an additional bedroom for a new family member, a dedicated leisure area such as a gym, or even a home office - crucial in recent times where remote work has become increasingly popular.  

Additionally, house extensions allow homeowners to tailor their living arrangements to suit their evolving needs without the hassle of relocating. And it's not just about immediate needs - there's a long-term perspective as well. Home extensions can significantly enhance the property's value, creating a solid investment for the future.

There’s a few things to consider when starting to think about a house extension, but we’re here to help!


The cost of house extensions can vary significantly depending on specific factors.

For instance, a garage conversion, one of the more economical options, typically ranges from £8.5k - £20k. However, more major undertakings such as a basement extension can cost more, ranging from £100k to £160k.

More broadly, a basic extension measuring around 20m2 carries an average UK cost of £48,000. In contrast, a larger extension of approximately 50m2 cost up to £120,000 on average.

It's important to note that these costs are dependent on several factors, including the size of the extension, the materials used, and the cost of labour.

Therefore, careful budget planning and consideration of these elements are crucial when deciding to start a home extension project. Thankfully, you’ll get to know us on a first name basis, our experts are on hand to ensure you get the materials you need for your project, as and when you need them. Ensuring that you can concentrate on the important elements.

The most expensive part of a house extension is often the foundation work and structural alterations required to support the new space. Labour costs will depend on the complexity of the project and the professionals needed to execute it. However, we can help with that, we have a Hire service at selected MKM branches, this allows you to hire the machinery and tools needed to get the job done, making lighter work of your project. Find out if there’s a branch providing Hire services near you.

What’s the cheapest way to extend a house and it still be safe and of good quality?

In general, a single storey extension will cost less than a multi-storey extension. Keeping costs down by choosing a simple design, using permitted development rights to avoid the need for planning permission and sourcing materials and labour locally, can all assist with costs.

Our branch staff are a knowledgeable bunch, why not pop into your local branch and discuss your project requirements over a cuppa and talk all things material. What’s more, you can get everything you need in one place, that’s from bricks and blocks to windows and doors, right through to plumbing and electrical. Once the extension is built, we can even design your new kitchen or bathroom for free if required, we’re always on hand to take the load off so that you can concentrate on what matters to you.

How long should an extension take to build?

An extension can take anywhere from 7 – 15 months to build. This depends on the size and scope of the extension. If you’re a homeowner, once you have considered what your extension will be used for and what you will need the extra room for, you will know if you need a single storey extension or two-storey extension.

Extension or conservatory?

If homeowners are looking to enjoy a few sunny afternoons or simply extend the living or dining space, then a conservatory is a good low-cost choice. However, if the aim is to provide a wide range of uses and even add a bedroom, then an extension is the way to go.

Whilst a conservatory adds much smaller value to a home and cost on average between £5,000 - £10,000 (much more complex designs can cost up to £30,000) an extension can provide much greater flexibility in terms of space and added value, with a larger price tag depending on the needs of the customer.  

Is it worth building an extension, what value will this bring?

Nationwide’s research on the value of home improvements to an average three-bedroom house found that an extension that creates a double bedroom and en-suite can increase the value of a property by 23%. There’s a growing need in the UK for larger houses with more room as families grow, so any extra space added is sure to increase the value.

Types of home extension:

  • Single storey
  • Multi Storey
  • Semi-detached extensions
  • Kitchen extension
  • Bathroom extension
  • Garage conversion
  • Loft conversion
  • Basement extensions

Building Regulations

Before any work is carried out, a builder, architect or surveyor must submit full plans of approval. Some examples of what building regulations cover, include:

  • Structural stability of a building.
  • Correct Fire Safety measures including fire escapes.
  • Ensuring a building is water and weather tight.
  • The use of toxic substances in cavity fill insulation systems.
  • Levels of sound insulation between buildings and rooms.
  • Technical design standards for sanitary pipework and drainage.
  • Underpinning the foundations of a building.
  • The design, installation, inspection and testing of electrical installations.

Building Regulations and planning permission

Prior to starting work, check with the Local Planning Authority to make them aware and to understand any considerations. Please be aware that England and Scotland have different requirements.

For example, when it comes down to materials that will be used for the extension, it may be important to ensure they are of a similar appearance to the rest of the property. This is where we come in, pop into the branch and allow us to help match the bricks and other features as best we can.

Two-storey extensions:

If you are planning on adding a two-storey extension to a home but you’re not sure if you want to tackle the obstacle of planning permission then do your research and explore the building regulations.

In most cases, planning applications are decided within 8 weeks. Find out more on the government website here.

Do I need to consider neighbours when planning on building an extension?

When planning an extension, it is always courteous for the homeowner to explain to neighbours that an extension is being planned, especially if the home is attached to theirs on either side, or both.

The Party Wall Act 1996 is a procedure to follow when something is being built that involves a ‘party wall’ that separates buildings belonging to different owners but can include garden walls built aside a boundary.

The act permits owners to carry our certain works, whilst at the same time protecting the interest of anyone else who may be affected to avoid or minimise disputes caused by certain works. The act also requires that, where the adjoining owner does not agree in wring to the works, a surveyor will weigh out which if the works can go ahead.

Works can include, but are not limited to:

  • cutting into a wall to take the bearing of a beam, for example for a loft conversion.
  • inserting a damp proof course, even if only to the homeowner’s side of a party wall
  • raising a party wall and, if necessary, cutting off any objects preventing this from happening
  • demolishing and rebuilding a party wall
  • underpinning a party wall or part of a party wall
  • weathering the junction of adjoining walls or buildings by cutting a flashing into an adjoining building
  • excavating foundations within three metres of a neighbour’s structure and lower than its foundations
  • excavating foundations within six metres of a neighbour’s structure and below a line drawn down at 45° from the bottom of its foundations.

Get to know! Key Acronyms for Development Permissions:

OPP – Outline Planning Permission

Outline Planning Permission (OPP) pertains to the preliminary approval for the fundamental development concept on a specific site. This authorisation allows for the subsequent definition of details such as sizes, materials, and access, at a later stage. It is essential to note that OPP alone does not grant the authority to commence construction. A supplementary application for full planning permission is still required. The validity of OPP typically spans three years, after which a reapplication becomes necessary.

FPP – Full Planning Permission

Full Planning Permission (FPP), also recognised as detailed planning permission, provides comprehensive details regarding the construction project. This includes specific dimensions, room layouts, and building materials. Upon the grant of FPP, construction can commence immediately. However, it is not uncommon to receive conditions of approval that must be adhered to throughout the project. Like OPP, FPP generally remains valid for a duration of three years.

Don’t forget these essential aspects before you dive into your new project.


How Much Will A House Extension Cost In 2023? | Checkatrade

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Prior approval - Extensions - Planning Portal


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