Costs Involved in a House Extension

House Extension Costs

The Basics

Deciding whether an extension is right for your home often feels overwhelming. On one hand, there are some great benefits to be had from extending your home- adding value to your property, the scope for creativity and the costs involved being cheaper than moving house being just some of them. But then, on the other hand, they can be stressful and expensive to complete and it’s worth bearing in mind you may not get exactly what you want.

With this in mind, an extension can feel like a daunting task – but it doesn’t have to be. It will seem like there are lots of decisions to make before you start from the rules and regulations to hiring the right builder for the job but the main thing to consider first are the costs. 

Once you have worked out your budget against the estimation of funds needed, you’ll be better placed to get started and look for a quote. 

Extension costs can vary widely depending on factors like size and how you intend to use it. Smaller, simple extensions tend to be the most affordable, but it doesn’t mean other, more complex options are out of scope.

Here are some examples of popular extension types and their estimated costs, including budget, mid-range and luxury, along with the time required to complete them:

Single-storey Extension £1,000 to £1,600 £1,700 to £2,000 £2,200 to £4,000 and up 8 to 10 weeks
Two-storey Extension £1,200 to £1,900 £2,000 to £2,200 £2,300 to £4,000 and up 12 to 16 weeks
Side Return Extension £1,500 to £1,900 £1,900 to £2,200 £2,200 to £2,500 10 to 12 weeks
Glass Extension £1,350 to £1,950 £1,800 to £2,300 £3,000 and up 10 to 12 weeks
Flat Pack Extension £750 to £1,440 £1,275 to £1,800 £1,650 to £3,600 1 to 2 weeks


It’s important to note that these estimates are just for the shell of the extension and do not include fittings and fixtures as these are elements you would decide on yourself from your chosen suppliers.

However, these do include VAT as most builders who undertake extension work are VAT registered so will include this in their quotes.

What about kitchen and bathroom extensions?

Kitchen and bathroom extensions are often the most popular choice for people considering an extension on their home. However, they can be a little more complicated as they require planning permission. The planning permission is to account for the plumbing and installation. It goes without saying that to make sure your extension is safe and durable this needs to be done properly by a reputable supplier. 

A bathroom extension will generally add an extra £5,000 to the above extension costs but again this doesn’t include fittings and fixtures (your choice of bathroom suite for example will be in addition to this.)

A kitchen extension will often be double this, with around an additional £10,000 to add for the building of your shell before fittings.

For example, for a 20 square metre kitchen extension, you should allow an extra £2,600 to £6,200 on top of the building costs if you’re on a strict budget (including labour).

Should your budget be higher, you could pay anywhere from £5,600 to £12,000 on top of the cost of the extension.

Bathroom extensions are cheaper as they tend to be smaller but it’s worth bearing in mind that the average costs of fitting a new bathroom suite range from £2,750 to £7,000 for a high specification finish.

What makes an extension more expensive?

Size shape and height

Naturally, the bigger or taller you go, the more costs involved. The type of materials and their level of premium will also affect this, e.g.  Brick face, timber clad, glass and steel fabrication. 


Hiring a surveyor, gaining planning permission and additional aspects like party wall agreements, and building regs approval will incur extra time and money but could be essential for you to be able to proceed with your project. 

Trees and groundworks

It’s very important to bear in mind that a lot of trees are protected by tree preservation orders and need to be taken into account when requesting planning permission. Groundworks like digging a foundation or altering drainage will also add to your total. Planning to build on uneven terrain will be more expensive than an existing flat surface. 

Windows, doors, fittings and fixtures. 

These finishing touches all add up and windows and doors can be particularly expensive (though unfortunately essential to most projects) 

Simple touches like painted walls and carpeted floors or basic laminate tend to be budget-friendly but more bespoke touches such as luxury flooring and tiling will come at a premium. 


As with most things, location plays a huge part in the price. Unfortunately living somewhere like London or a major city will make your extension more expensive and should be taken into account when reviewing quotes and prices. 

Finding a builder

Builder’s rates will vary and finding the professional most suitable for your project (and of course budget) is a personal decision. When looking for a builder you might find it useful to consider recommendations from people who have carried out similar work to you on their properties. Aim to review three to five quotes from potential contractors to see what works best for you. 

Cutting Costs

To keep the costs of your extension as budget-friendly as possible, planning is key. Don’t be tempted to scrimp on the structural integrity of your extension. Be sure to hire a surveyor and structural engineer or architect to draw your plans or you may find this costs you significantly more should things go wrong later down the line. Instead, you can save on fitting costs. Go for off-the-shelf products from high street retailers over bespoke items and take advantage of online marketplaces, sales and preloved items to keep costs down.

Next Steps

Once you’ve explored and calculated the costs of all these factors you should find yourself better placed to decide if a home extension is the right move for you and take the next steps to get your project off the ground.  With 30 years of experience in the industry, our project managers are on hand to ease and facilitate the manufacture and delivery of steels for your project and offer a 48-hour turnaround time from agreed drawings. Get in touch with us on 01384 389096 or email and one of our friendly and professional team will be happy to advise further.