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The cost of a rear extension

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The cost of a rear extension

Rear extensions are one of the most popular types of conversion and extend backwards from the rear of your home into your garden space. This is popular since it is less likely to require planning permission, does not impact the curb appeal of your property, and makes good use of your land. A typical rear extension can cost from £1,500 per m2 and the average cost of a small rear extension of 15m2 in total is usually between £15,000-£20,000.

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What is a rear extension?

A rear extension is a conversion built outwards from the back of your property, usually into your garden space. The cost of your extension will depend on many factors such as how large your build project is, the extent of professional costs involved, what materials you choose and what you will use your new space for.

Extending outwards can be either single storey or double-height, with the most popular being a single-storey extension. This offers expanded living space, the option of creating a new room or extending the size of an existing one, and capitalising on the capacity of your property to grow as your requirements change.

Often an extension is an alternative to moving home and can increase the value of your house as well as improving your quality of living.

Why are rear extensions popular?

Choosing to extend your home can sound like a costly and long-winded process; however, this is a cost-efficient method of increasing the amount of living space within your home, whilst also increasing its value.

Particularly for period properties or in built-up urban areas, an extension makes the best use of the space available. A rear extension usually extends across the full width of your property, can increase the natural light within your ground floor, and can add anything from 6% to 20% to the value of your home.

Benefits of rear extension include:

  • Versatility of use
  • Options for small budgets or large investments
  • Can be built without planning permission depending on the design
  • Creates open-plan living spaces
  • Increases the value of your home
  • Improves access to natural light
  • The fairly simple build process

What are the uses of a rear extension?

Building outwards from the back of your property can create living space for any number of purposes, and can even be double-height to create an extra bedroom, bathroom or extension on your upper floor.

Typical uses of a ground floor extension include:

  • Increasing the size of your kitchen or living room
  • Building a standalone playroom, home office or sunroom
  • Changing your layout to accommodate open plan living

How much does a rear extension cost?

Budgeting for a rear extension depends on many factors but starts at around £1,500 per m2 up to £2,500 per m2 and above for large, complex or unusual builds.

Average costs for rear extensions are:

  • £15,000-£20,000 for a small extension of around 15m2
  • £30,000-£40,000 for a medium extension of around 25m2
  • £50,000-£60,000 for a large extension of around 50m2

These costs all depend on the use of your extension, what it is built from, and how intricate or complex your design is.

How long does it take to build a rear extension?

The time taken to build your extension will depend on all of the factors involved in your design. Typically, the professional design process and planning permission application time – where required – will take just as long as the build process itself.

Most extensions are completed in around four to six months and longer for a larger build, with around the same timescale required for the design and planning process. To allow plenty of time, it is best to allow around one year in total for the complete project.

What factors impact the cost of a rear extension?

Since a rear extension can be built for many purposes with many different designs, this will impact your overall budget.

Choosing a new kitchen or bathroom will be more expensive since there will be the additional cost of the units, and knocking through internal walls to create open plan living will include the cost of internal works.

As an indicative guide, the cost implications of rear extensions are:

  1. Glazing: The more glazing you have, the more expensive; but for a sunroom or conservatory area you can opt for standard size glazing which is cheaper than custom-designed frames and glass. Double-glazing costs around £300 per window. Triple glazing is more advanced and energy-efficient, and costs around £400 per window.
  2. Bathroom: Building a new bathroom includes a sink, shower, bath and toilet depending on whether it is a downstairs WC, shower room or complete bathroom. The average cost of a new bathroom is £4,500 depending on what fittings you need and what materials you choose.
  3. Kitchen: A popular use of a ground floor extension is to build a larger kitchen area. This budget depends on your design – for example, a laminate worktop costs £30-£40 per m2, whereas a granite worktop can cost up to £400 per m2.
  4. Doors: New external doors cost around £300 for steel doors as the cheapest option and around £1,600 for a set of patio doors. Other options include softwood, fibreglass and sliding doors.
  5. Lighting: As an alternative to standard lighting, smart lighting can cost up to £500 – this is more expensive, but is energy efficient and can be pre-set to avoid unnecessary electricity usage and pay for itself over the long-term.
  6. Heating: If your extension requires a new gas boiler and pipework, you should budget around £2,000-£3,000. Underfloor heating is increasingly popular, and costs from £60-£100 per m2.
  7. Electrics: Most rear extensions will require electrical work, and the cost will be dependant on how extensive this work is, and what electrical points you require.
  8. Flooring: Flooring costs vary significantly between tiling, wooden floors, laminates and carpets.
  9. Walls: Plastering tends to cost around £25 per m2, and if you choose to have your extension professionally decorated should budget for the time taken and the cost of your materials. The cost will depend on what sort of paints you choose, or whether you decide to use wallpaper – eco-paper tends to costs slightly more but does not release any toxins into your home so there are a wealth of choices to decide between.

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Frequently asked questions

Do I need planning permission for a rear extension?

Some rear extensions do not require planning permission; whilst this is not a particularly expensive process, costing £206 for an extension application, it can take several months and incur other expenses such as design fees.

If your rear extension falls within a specific set of criteria, you can build under permitted development (PD) – please note that Building Regulations still apply and that it is always wise to check with your local planning authority before making any assumptions.

Examples of builds that usually do not require planning permission:

  • Do not extend more than 8 metres to the rear for detached homes
  • Are not more than 4 metres high, or are no higher than your existing property
  • Do not fall within 7 metres of a boundary for a double-height extension

What other permissions might I need for a rear extension?

If your home is terraced or semi-detached your extension is more likely to come within proximity of a boundary wall with your neighbours; in which case you will likely require planning permission.

You may also require a Party Wall Agreement, which exists between you and your neighbours to avoid any disputes. This sort of agreement covers homes with shared walls and boundaries and may require a professional party wall surveyor.

This process can cost around £300 for a simple agreement, but if extensive negotiations are required can be significantly more expensive and cost upwards of £1,500 per household. Note that the homeowner intending to build is usually expected to cover these costs, so it is advisable to approach your neighbours about your extension plans before any work begins.

Do Building Regulations apply to rear extensions?

Any building work must be compliant with Building Regulations, even for small and straightforward builds. Usually, this involves applying to your local authority either through a building notice or full plan submission.

A building notice informs of your intention to build and must be delivered with a minimum of 48 hours notice before work begins. A full plan submission is required for more complex projects.

The local authority will send a Building Regulations inspector at intervals to confirm that that build is safe and compliant with regulations.

What professionals are involved in a rear extension build?

The exact timescale and professionals involved in each stage will depend on-site access, materials and the design of your extension, but will typically consist of the below tradespeople:

  • Builder: average hourly cost £65
  • Plumber: average hourly cost £55
  • Electrician: average hourly cost £50
  • Joiner: average hourly cost £12
  • Structural engineer: average hourly cost £50
  • Architect: average hourly cost £50
  • Glazier: average hourly cost £15
  • Roofer: average hourly cost £50
  • Kitchen fitter: average hourly cost £40

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