Guide to home extension costs
The Build It Estimating Service’s Adrian Wild takes a look at what you can expect to pay to complete a typical conservatory, flat roof extension or pitched addition to your home
The low-down on what you can expect to pay when building a typical single-storey addition
If you have enough garden amenity, building an extension can be a great way to add space and value to your home. According to the Office for National Statistics, a typical 20m2 extension would increase the market worth of a property in England or Wales by an average of £47,260. The examples featured here offer a good indication of what you can expect to pay for different types of extension constructed to a fairly standard level of quality, using a main contractor.
Conservatories
One of the most affordable routes to gaining living space is to install a conservatory. Our model project, based on a design by Crystal Direct, would add 20m2 of extra floor plan. Some conservatories of less than 30m2 are exempt from Building Regulations, which can speed up the process and cut out a few fees. In this case, we’re planning to tap into the main house’s heating system – so the project will need to conform.
The Build It Estimating Service prices the work at £22,864 including labour, plant, materials, contractor’s profit margin and VAT. This works out at £1,143 per m2 of floor space. The biggest outlay is the actual conservatory structure, at £6,304 before factoring in the builder’s fees and VAT. This will sit on 600mm dwarf walls. Generally speaking a conservatory is quicker to construct than an all-masonry extension.
Single-storey extensions
If you want maximum flexibility over the design and layout, a more traditionally-built extension might be the best bet. At the rear of the house, this kind of project is often undertaken to create spacious open-plan zones that connect well with the garden while offering all the convenience of a solid-walled room. This type of extension might be allowed without formal planning consent under permitted development rights (see www.self-build.co.uk/permitted-development) but will always require Building Regulations approval.
Option 1: Flat roof
Using the Build It Estimating Service, we’ve priced up a model 20m2 flat roof single-storey rear extension (sometimes known as a garden room) at £34,534 including labour, plant, materials, contractor’s profit margin and VAT (£1,767 per m2). The project features bifold doors and a roof lantern to give a light-filled zone, and could be ideal for a kitchen-diner or something similar.
Option 2: Apex roof
Flat coverings don’t suit every project. Sometimes a pitched roof extension is more appropriate to the style of the house, so we’ve also priced an apex design. We’d expect this to come in just over £1,700 more expensive at £36,266 including VAT (£1,813 per m2). This is largely down to the extra costs involved in erecting and finishing a gable wall inside and out.
MODEL COST GUIDE: EXTENSIONS
These benchmark costs are based on the construction of three different types of 20m2 extension – all 5m wide and projecting 4m back from the rear of the property. The prices include works such as slab foundations, knocking through to the existing building, brick and block construction and PVCU windows (the two standard extensions include a 3.6m run of bifold doors and four 1.2m x 1.35m windows).
The heating system and electrics will be extended from the main property. Extension projects are subject to VAT at the standard 20% rate. We have assumed a builder’s markup (profit) of 30%. The main exclusions are design charges, planning costs and fees, drainage, site cleaning, landscaping, floor finishes and internal fit-out.