How Much Does An Extension Cost?

If you’ve been wanting to increase the space you live, but don’t want to deal with the hassle of moving home and still love where you live, extending your house is a great alternative; and often far cheaper than moving to a bigger home.

If you’ve got the space, then adding an extension to your home can be a great way to improve your home with some extra usable space, all while increasing its value. It may seem like a simple feat to estimate the cost of your extension prior to booking a consultation, however there is no singular cost per extension. 

Size, location, specification, and build route will all affect how much your new extension will cost, but as a general rule; the building cost per m2 can be as little as £1000 for simple and straightforward conservatory style rear extensions.
How much your extension costs can be affected by things like:

  • How many storeys you’re going to build 
  • Whether it is large or small, and the shape of the extension
  • The build route you’ll take – how involved in the project you’ll be
  • The amount of glazing you’d like
  • What you’ll be using the extension for and what it needs to be fitted out with. A kitchen may need gas, a bathroom will need water, you may want underfloor heating, all will need electricity. 

However, as extensions can vary in size, style, and number of storeys – fairly basic single storey extensions that are fully integrated into the main house can begin at around £1,250 per m2. Depending on the location in the country, and how high-end your design is, you can expect this to rise to well over £2,500 per m2.

Do you Need Planning Permission for your Extension?

Depending on the size and specifications of your planned extension, you may need to obtain planning permission from your local government. However, as a guide, If your scheme fits within the parameters below, there’s a good chance it will be allowable as a permitted development:

  • Extensions must not exceed 50% of the total land area around the original house.
  • You cannot extend forward of the main elevation, or the side elevation where it fronts the highway.
  • Single-storey rear additions must not extend beyond the rear of the original house by more than 3m for terraces and semi-detached homes, or by 4m for detached houses.
  • Side extensions must be single storey with a maximum height of 4m and must be no greater than half the width of the original house.
  • Extensions within 2m of a boundary should have a maximum eaves height of no more than 3m.
  • Multi-storey extensions should have a maximum eaves and ridge height no taller than the existing house, with a matching roof pitch.
  • A multi-storey addition cannot extend beyond the rear wall of the original house by more than 3m or be within 7m of any boundary opposite the rear wall.
  • Any windows on upper floors of side elevations must be obscure-glazed and non-opening, unless the openable part is at least 1.7m above floor level.
  • You must use external materials that are similar in appearance to the existing house, unless you’re adding a conservatory.

To be doubly sure whether or not you need planning permission, and to check if this will apply to you, please read through the latest government legislation here. 

What Costs Should You Expect When Building an Extension?

As well as the specifications of your build, the construction costs, which can be dependent on things like soil type and glazing, the build route, and the trades, there are other costs to consider outside of the build itself. 

Here are some example costs you may need to factor in:

  • Architect fees 
  • Measure surveys and other surveys
  • Planning application (if needed)
  • Lawful Development Certificate (currently £103)
  • What the extension will be used for (and how much it will cost to fit it out)
  • Party Wall Agreement (if needed)
  • Insurance

How Much Does a Single-Storey Extension Cost?

If your plans only involve a fairly straightforward box-shaped extension, your budget should allow for between £1250 to £2000 per square metre. Depending where in the UK you’re hoping to build your extension, the price could increase exponentially if it is a high sought-after area. 

As a basic guide, for an excellent finish from top craftspeople, you should expect to pay up to 40% more than a standard finish. 

  • A 20m2 extension can cost £25,000 – £50,000.
  • A 30m2 extension ranges from £37,500 – £67,500.
  • A 50m2 extension could set you back between £62,500 – £112,500.

How Much Does a Two-Storey Extension Cost?

If you’re planning to build a double storey extension, it will definitely be cheaper in the long run to save your budget and build them at the same time; this will also save the extra hassle of dealing with two separate building events at different times. 

Two-storey builds do tend to be the most cost effective to build. Adding on considerable living space, and minimising the impact on garden space, a two-storey build won’t cost a huge amount more than a single storey extension due to the only extra costs being from the extra interior fixtures and finishes, walls, and floor joists.

A roof and foundations are going to be required whether you build a single or two-storey extension. Other additional costs are mainly for safe working at height – such as scaffolding and some additional structural support.

The cost of a side extension, for example, will depend largely on the size of the structure, the location of the build and the quality of the materials and finishes. A typical 60m2 two storey house extension can cost between £75,000 – £135,000.

How Much Does a Kitchen or Bathroom Extension Cost?

When you opt for a bathroom extension, you should note that there are two main types of bathroom extension for your home:

  • Adding a completely new bathroom to your home such as:
    – Adding an en suite
    – Adding a ground floor toilet
  • Expanding a bathroom: creating extra space in an existing bathroom that could be used to install a walk-in shower, extra sinks or other amenities.

Costs per M2 are typically similar to the estimated prices listed above, however, the cost of your bathroom extension will be higher due to the extra building work required, such as the installation of watermains, decorations, and other amenities you require. This added complexity compared to other rooms in your house causes bathrooms to be some of the most expensive types of extension. For this reason, you should budget an extra £2500 – £5000 to the total cost of any bathroom renovations you have planned. 

Kitchen extension costs

Kitchen extensions require even more specialist work compared to bathrooms, this means that your kitchen extension will be higher than a typical room extension. As well as needing plumbing installed, you’ll need to install gas and electricity; as well as additional costs if you opt to purchase a brand new boiler – or even move an existing boiler.

Due to the specialist additions required for a kitchen extension, we recommend budgeting an extra £10,000 – £25,000 to the average cost of your kitchen extension in order to afford the decorations and amenities you’re wanting installed. 

Are You Looking For an Extension on Your Property?

The Guild of Master Craftsmen have members that are expert builders working across the whole of the UK. take a look at our builders category to find a member near you.