The Cost of a Rear Extension

All you need to know about having an extension built to the rear of your home including costs of materials, labour and time frames.

The Cost Of a Rear Extension

What the job entails

Building an extension can be a cost-effective alternative to moving home when more living space is needed, extending the existing home can often work out to be cheaper than moving once you factor in all costs such as solicitors fees, removals and stamp duty. There are many ways to extend a property, but extending into the rear garden is probably the most popular, for the simple reason that most homes have more available space to the rear.

A well planned and professional extension should more than pay for itself in terms of adding value to the property, but adding an extension is a project that will be beyond all but the most experienced DIY enthusiasts, so finding a reputable and professional builder is essential. In addition, an architect will be required and the local authority planning department will need to be consulted.

Building a rear extension is a big, messy project, so will involve a lot of upheaval including extensive foundation work in the rear garden. The first step with an extinction is the planning and design which should be undertaken by professionals, or as a minimum with professional advice from an architect and/ or a structural engineer. The work can be managed by yourself or carried out by a design and build company. Managing the project yourself will, of course, save you money but then you have to hire subcontractors for each part of the project and take on the role of project manager. It is much easier all round if you can move out while the building works are in progress, however, this is not always possible.

While you have builders and other tradesmen on site, it will usually work out cheaper if you have them attend to any other jobs that may need doing at the same time. Also, if the rear extension is planned to match the existing building, then you may find that you need to clean or paint the exterior walls as the brand new construction will make them look grimy and dirty.

To make sure a planned extension add value to your property, you need to put a lot of thought into it even if it is a simple, small extension. There are many factors which affect the cost of a planned extension, but as a rule of thumb, extensions will cost somewhere between £1,000 and £2,000 per square metre. For large jobs like extensions, builders generally prefer being paid a deposit with further stage payments made for each completed phase of the project. However, you should always try and minimise any upfront payments with building projects.

Planning permission may or may not be required depending on the size of the extension and the existing property. The permitted development rights allow home extensions to be added without planning permission as long as they meet certain criteria. Under permitted development rights you can extend a detached house by up to 8 metres to the rear as long as it is not higher than 4 metres or higher than the existing building. If the rear extension is a two-story extension, it must be built of a similar material to the existing building and be no closer than 7 metres to the rear boundary. But in all cases you are responsible for permissions and consents, not the builder, so always double check with the local authority.

Building an extension is a large project involving many different tradesmen and often subject to some quite complex legislation. Failure to take professional advice will likely end in financial disaster. The vast majority of problems which occur with extensions involve poor planning, poor design or cowboy builders. So, if you want to avoid expensive and possibly dangerous problems, use an architect or structural engineer to help design the extension and give advice on the process and the costs, then hire a reputable builder to carry out the work for you.

Ready to get a quote from local tradesmen?

Post your job in minutes and get quotes from local and reliable trades. It's FREE. No obligations.

Get a Quote

The Cost of a Rear Extension

To provide accurate pricing information we have contacted local and national companies, plus have used various online quotation tools. The figures provided should give a good rough idea of the costs involved if you are considering a rear extension, but of course you will need to gather detailed quotes from tradespeople to get an exact price for your requirements. You can get up to 3 free quotes from Local tradespeople by clicking the "Get a Quote" button.

In order to give accurate costs for a rear extensions, we've gathered dozens of quotes from small and large companies across the UK. These figures should give homeowners a ball-park figure of the costs of having a rear extension built, however these numbers can vary quite considerably so always compare quotes yourself before getting the work done.

The average material cost for a rear extension depends on numerous factors such as the size of the extension, the quality of the materials and the types of materials. For example, if you're looking to add a kitchen then you should add an additional £3000-£10,000 to your costs. Alternatively, you could be looking to add a second living room, in which case additional costs are going to be focused around furniture.

The average building specialist will usually charge around £1000-£1500 per m2. For a rear extension you will need several trades to complete your job. An architect and a structural engineer are going to be needed to help with the planning applications, reports and to follow relevant building regulations. A builder and roofer to complete all of the structural work, depending on your design. An electrician to install the cabling, lights, switches, sockets and potentially heating. A heating specialist or plumber for radiators, pipe work and installation of a cooker or fire if a kitchen or living room is in the plans. A plasterer, decorator, tiler and/or flooring specialist for the walls, ceiling and floors. A joiner for kitchen fitting or any bespoke furniture such as boiler encasement. A culmination of all the trades you are going to need as well as waiting times whilst things are in the making/drying can easily see the project taking a few months.

Below are some estimated costs of a rear extension:

Extension Size Avg. Cost Duration
Small Rear Extension - 15 m2 £15,000 - £20,000 8-10 weeks
Medium Rear Extension - 25 m2 £30,000 - £40,000 10-12 weeks
Large Rear Extension - 50 m2 £50,000 - £60,000 10-12 weeks

Cost Breakdown

Individual costs of a medium rear extension at 25 m2 - Total Cost: £30,000






Waste Removal


Typically you can expect the whole process to take in the region of 3 months. this includes building regulations approval which can take two weeks.
Expect to pay in the region of £1,500 to cover architects fees, planning permission application and building control inspection.
No, this will depend on the available garden space and the current regulations.
If you have a party wall that you will have to inform your neighbours about the planned works and you will need a surveyor to issue notices on your behalf and to inspect the Shared wall. This is a completely separate process to the planning permission application.
On average you can expect a timescale in the region of 10 to 12 weeks, but this will, of course, vary greatly depending on the size and specification of the extension.
No matter how professional the builders, a rear extension can be disruptive and stressful! If the budget allows, it is best to avoid the disruption by simply moving out while the works are in progress. But reputable builders will do their best to keep disruption to a minimum by using dust sheets and even sometimes building temporary stud walls and kitchen areas!
Yes, written permission will be required from your freeholder before extending the property.
Permissions and consents are your responsibility, however, professional building companies will often be able to take care of this for you. Larger building companies will have their own architects and surveyors, plus they will often have a good working relationship with the local authority.