Extending your house: what to think about first

Short answer: how to extend without expensive mistakes

Start with the layout, not the square metres. A good extension solves one clear need – a bigger kitchen, another bedroom, a home office – and joins the existing house so the flow gets better, not worse. Then check whether it fits permitted development or needs planning permission.

The costliest problems are the ones found late: a room that goes dark, new foundations settling against old, a roof that will not seal. Draw several options before you talk to builders.

Do you need planning permission for an extension?

Many single-storey rear and side extensions fall under permitted development, within limits on depth, height and materials. Conservation areas, listed buildings and properties with removed rights are different, and Building Regulations apply either way. The Party Wall Act may apply near a boundary in England and Wales. Check with your council – a Lawful Development Certificate gives certainty.

• Check permitted development limits for your house.
• Ask whether you are in a conservation area or have removed rights.
• Check Building Regulations and Party Wall needs.
• Consider a Lawful Development Certificate.

Build out, up or over the garage?

Building out is usually simplest but takes garden. Building up saves ground but needs a staircase that eats into the floor below, and the structure and foundations must carry the load. Over a garage the floor exists, but it often needs strengthening, insulation and fire separation. Ask a structural engineer before you choose.

How does the new part join the existing layout?

Decide where the opening between old and new goes – it sets the whole flow. A dining area should sit next to the kitchen, a bedroom near a bathroom. Avoid turning an existing room into a dark corridor. Plan what happens to rooms that change use: the old kitchen may become a utility or study.

Light and orientation

An extension often takes light from the room it is built against. Draw windows in both parts and see which rooms lose daylight. Rooflights or a higher ceiling in the new part can save it. Put a garden room or dining area towards evening sun and think about overlooking.

Foundations and settlement

New foundations often settle differently from old ones. The joint between them needs planning, and Building Control will want to see the foundation design. Cracks at the junction are expensive to fix later.

Roof and wall junctions

Where the new roof meets the old, leaks and damp traps appear easily. Choose pitch and height so the junction is simple – sometimes a lean-to or flat roof works better than matching the ridge. Matching materials is often expected under permitted development.

Heating, ventilation, electrics and plumbing

Check whether the boiler or heat pump, consumer unit and ventilation can cope with the extra space. New bathrooms are cheapest near existing soil stacks. Electrical work falls under Part P.

Build time and living on site

You can often stay, but expect dust, noise and a period with a temporary wall where the opening will go. Plan where the kitchen lives meanwhile and break through to the house as late as possible.

The plot and your neighbours

An extension moves the house closer to the boundary and affects access, parking, drainage and garden. Draw it on the site plan early and talk to neighbours, especially if party wall notices are needed.

Is an extension worth it?

It can add value, but not always as much as it costs. Extra bedrooms, bathrooms and a well-sized kitchen usually pay back best. Compare with moving, and get quotes on the same drawing.

Common extension mistakes

• An existing room goes dark or becomes a corridor.
• Foundations at the junction settle – cracks later.
• Roof junction traps water.
• Heating and electrics cannot cope.
• New bathroom far from the soil stack.
• Quotes requested without a drawing.
• No plan for living through the build.

Checklist before you extend

Save this list and tick it off before you apply or get quotes.

• Need defined: what should the space solve?
• Permitted development or planning confirmed.
• Out, up or over the garage compared.
• Existing and new drawn to scale.
• Light and flow in old rooms checked.
• Ground and structure assessed.
• Heating, electrics and plumbing checked.
• Plan for living on site.
• Site plan, plans, elevations and section ready.

Draw and test the layout before talking to builders

Draw the existing house, add the new part and furnish it to real size. Try two or three options – out, wrap-around and up – and compare flow, light and what happens to the old rooms. With a finished drawing every builder quotes the same job.

Frequently asked questions