How to Plan a Single-Storey Rear Extension

Begin with the brief, not the budget

Most single-storey rear extensions solve a specific problem: a gloomy kitchen, a dining table squeezed against a wall, a garden you can only see through a small window. Before you think about square metres, spend an evening writing down how you want the space to work. Where does the sun land in the afternoon? Do you cook with people gathered around, or do you want the kitchen to remain a separate room? Is this a garden room that happens to open off the kitchen, or an enlarged kitchen with a view out?

Those answers shape everything that follows – the depth of the plan, the quantity of glazing, whether you need steel. A well-arranged 3m by 5m extension will feel more generous than a badly planned 4m by 6m one. Sketch it, debate it, then hand the agreed brief to your designer. A clear brief saves more money than any clever cost-cutting later.

Setting a budget that survives contact with reality

Across most of the UK, a properly built single-storey extension currently comes in somewhere between £2,000 and £3,000 per square metre, with London and the South East at the upper end or beyond. A 20sqm extension therefore means roughly £40,000 to £60,000 before you have chosen a single tile. Structural steel, large glazed openings, awkward site access and the re-routing of drains all push that figure higher.

Build these into your total from the outset:

• Professional fees: measured survey, architectural drawings and structural calculations, often £2,000–£5,000 combined for a straightforward project.
• Statutory fees: the householder planning application fee (check the current figure with your local authority) and Building Regulations charges.
• Party Wall costs: if you are excavating within three or six metres of a neighbour's structure, you may need a surveyor's award under the Party Wall etc. Act 1996.
• A contingency of 10–15%. This is not padding. It pays for the drain nobody knew about, the rotten lintel, the foundations that had to go deeper than the drawings suggested.

Keep your kitchen, flooring and lighting as a separate line in the spreadsheet. That is where budgets most often slip without anyone noticing.

Choosing materials with your eyes open

Planning authorities care far more than many homeowners expect about how an extension sits against the original house. Matching brick, or a tonal match with a contrast panel, usually reads better than a sudden change of material. Render, timber cladding and brick slips all have their place, but each has maintenance implications: timber needs recoating, render can crack, brick ages gracefully.

For the roof, a flat design with a high-performance GRP or single-ply membrane is often the simplest way to keep a low profile and avoid stealing light from upstairs windows, though a pitched roof finished in matching tiles can look more traditional. Roof lanterns and large flat rooflights transform darker interiors, but they are expensive, so place them where they will actually catch light.

Glazing is the single biggest visual decision. Aluminium frames are slim and durable, timber suits period properties, and modern uPVC is far better than its reputation suggests. Building Regulations set a maximum U-value for new glazing – typically around 1.6 W/m²K for extensions – so ask suppliers for certified performance figures rather than taking a verbal assurance. Triple glazing is rarely worth the extra cost in a south-facing garden room; spend the money on better doors instead.

Drawings and technical information

Before anything is submitted, a measured survey of the existing house establishes the truth about levels, boundaries and drainage. From that, your designer produces existing and proposed floor plans, elevations and a site or block plan showing the extension in relation to boundaries. These are normally drawn at 1:100 or 1:50, and local authorities are strict about accuracy – an extension that appears to overhang a boundary on paper will attract questions.

Sections are sometimes requested, particularly where roof heights or neighbouring windows are sensitive. If your property sits in a conservation area, is listed, or lies in a flood zone, expect additional information to be required. Good drawings do more than satisfy a case officer: they give your builder something unambiguous to price and build from.

Planning permission or permitted development?

Many single-storey rear extensions fall under permitted development, which means no planning application is needed, provided you stay within limits. For most houses the extension can project up to 3m from the original rear wall, or 4m for a detached house, with a maximum height of 4m and eaves no higher than the existing. Materials must be of a similar appearance, and the extension must not cover more than half the land around the original house.

A prior approval route allows larger single-storey extensions – up to 6m, or 8m for detached homes – but neighbours are consulted and the local authority can refuse on amenity grounds. Flats, maisonettes, listed buildings, conservation areas and properties affected by an Article 4 direction generally lose these rights.

Where full planning permission is needed, a householder application is usually determined within eight weeks in England. Expect neighbour consultation and, sometimes, a request for amended drawings. Planning consent and Building Regulations approval are entirely separate: you will need the latter before work starts, whatever the planning outcome.

Getting the order right

The sequence that causes fewest headaches is: agree the brief, appoint a designer, survey, draw, confirm your planning route, then approach builders with a genuinely detailed package. Trying to price a vague idea is how homeowners end up comparing quotations that describe completely different projects. Do the thinking and the drawings first, and the build itself becomes a far calmer affair.