How Much Does It Cost To Build An Annex

How much does it cost to build an annex?


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Last updated Thursday 14th May 2020

Bailyannexe 2

If you’ve decided that erecting an annex is the right option for your home, make sure you’ve got your budget nailed down before you do so. It’s always good to get all the potential costs out in the open at the very start of your project, so you don’t leave yourself open to that oh so terrifying mid-build crisis.

You could be setting yourself back a total of £18,000 for a medium sized annex of 15 square metres.

Hiring professionals

Before you begin your build, you’ll need to get an architect on board in order to have a well designed annex. If you choose not to go down this route and advance without architectural plans, there’s a good chance you’ll be incurring future costs that may arise when measurements are off, or planning is not approved.

Speak to one of our Resi architects, to find out how we can help.

You’ll also have to invest in a structural engineer - to ensure the build is structurally sound, a surveyor - to assess your property’s boundary lines, and possibly a party wall solicitor - to settle any neighbourly disputes regarding your shared wall. You'll find these costs building up throughout your project.

Once plans have been finalised and you’re ready to build, you’ll need to hire a project manager and contractor. Of course, this will come at a price, however we highly advise not to go straight for the cheapest option. Look around and make sure you obtain at least three quotes before you make your final choice. The quality of the construction company and project manager could make or break the build (quite literally!).

However, if you’re good with your hands, doing most of the construction work yourself could shave 30-40% off your bill.

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External structure

This cost is very dependent on what quality of annex you are intending to build. Higher quality, for example brickwork with good insulation, is going to be more expensive than a basic wood frame. What material you’re looking to use is up to you; it's possible to save on these by sourcing them yourself, instead of having your project manager or contractor do so on your behalf.

Regarding the roof, most annexes feature a flat roof. However, if you’re thinking of opting for a pitched roof, this will increase the price. Pitched roofs will usually be 25% more expensive, due to the differing materials and extra time it takes to build this style.

Glazing and doors

There are many different options for your choice of doors, and which one you decide on depends on your personal taste and budget.

The three main types are:

  • French doors: traditional doors opening up outwards, containing a number of small windows panes. These are usually the most cost effective option, as they are generally within the region of £800 per set.
  • Bi-fold doors: tall, thin panels which concertina outwards to the side of your doorway. These can cost anywhere from £1,500 to £2,500, depending on the quality and number of panels you go for.
  • Sliding doors: two large panes of glass, which move behind each other to allow access to the outdoors. Good quality sliding doors of 4 panels would normally cost around £4,000.

When taking these prices into consideration, it’s important to remember that your choice of materials can make these prices fluctuate. For example, going for white uPVC frames will be considerably cheaper than good quality aluminium frames.

What type of glazing you have for these doors, and windows throughout the annex, may be another cost consideration. While double glazing is a great option for improving insulation and saving on energy bills, the initial fitting and purchase cost of roughly £500 per window is often too expensive for many. If this is the case, why not go for secondary glazing? While not quite as effective an insulator, you can expect to pay about 1/5th of the price of double glazing.

Find out about your different glazing options here.


It can be really easy to get swept away with all the design aspects and forget about costs that aren’t quite as physical - the utilities.

The costs of services such as water and electricity are dependent on how far your annex sits from the main house. Usually, underground services will have to be installed between the two properties, which could cost up to £1,000. You’ll also need to connect the electricity supply, which will mean hiring a licensed electrician. In some cases, modifications may be need to be made to the main house electricity supply in order to feed the new annex - which will also come at a cost.


What has been provided are the basic costs of erecting an annex. However, the rooms included in your new build and how they're furnished will affect your budget - for example, a fully fitted kitchen will be much more expensive than a basic lounge area.

However, it’s important to remember not to stretch your budget too far when installing furnishings. Generally, a more effective route would be to invest in a good quality build, as opposed to high end installations, in order to increase the resale value of your property.

Thinking of building an annex? Book a free consultation call with one of our expert advisers to find out about the next steps of your project.

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