Planning a Single-Storey Extension for Your Home

Do You Need Planning Permission?

Many single-storey extensions can be built under permitted development rights, which means you may not need a full planning application. But the rules are strict and easy to trip over. For a rear extension, permitted development usually allows up to 3 metres beyond the original rear wall for terraced and semi-detached homes, or 4 metres for detached homes. The extension must not be more than 4 metres high, and the eaves must not be higher than the existing roof. Side extensions have different limits, and materials should match the existing house.

Your permitted development rights can be removed if you live in a conservation area, an Area of Outstanding Natural Beauty, a National Park, or if your home is listed. Some properties also have an Article 4 direction that takes away these rights. Before you assume anything, check with your local planning authority. A Lawful Development Certificate is a small investment that proves your extension is lawful – it is invaluable when you come to sell.

• Measure carefully: the original rear wall, not a later addition, is the reference point.
• Keep extension roof ridges below the highest part of your existing roof.
• Remember that decking, patios and outbuildings can also count towards permitted development limits.

Setting a Realistic Budget

A single-storey extension in the UK typically costs between £1,500 and £2,500 per square metre, depending on specification and location. A 20 square metre extension might therefore range from £30,000 to £50,000. That figure should include VAT, professional fees, and a contingency of at least 10 to 15 per cent. Too many homeowners forget the hidden costs: moving drains, upgrading the consumer unit, new flooring, decorating, and the cost of living without a kitchen for several weeks.

Get at least three itemised quotes from builders, not vague estimates. Ask what is excluded – skips, scaffolding, building control fees, and waste removal often appear as extras. Never pay a large deposit upfront. A fair payment schedule ties payments to completed milestones, such as foundations, roof, and first fix. Keep a written contract that sets out the scope, timeline, and payment terms.

Why a Structural Survey Matters

Even a modest extension places new loads on your existing home. A structural survey or at least a report from a structural engineer will check the ground conditions, foundations, and drainage runs. In older properties, this can reveal the need for piling, which can add thousands to your budget. It might also uncover asbestos in roof sheets, soffits, or floor tiles – a job for a licensed contractor, not a general builder.

If you share a wall with a neighbour, the Party Wall etc. Act 1996 may apply. You must serve notice and, if the neighbour dissents, appoint a party wall surveyor. Ignoring this can lead to an injunction and costly delays. A survey is not an unnecessary expense; it is your best defence against nasty surprises once the diggers arrive.

Getting the Design and Build Right

Appoint an architect or architectural designer early. They will prepare drawings for both planning and building regulations, and they can spot problems before they reach site. Think about thermal performance, glazing, and roof lanterns – a well-insulated extension with underfloor heating can be comfortable all year without huge bills. But specification affects cost. Bi-fold doors and large expanses of glass look wonderful but need expensive steel beams and careful detailing.

Choose your builder with care. Ask for references from similar projects and check they have public liability insurance and a valid waste carrier licence. A written contract should include a start date, completion date, and a defects period. Avoid making changes once work begins; each variation slows the build and inflates the final bill.

Staying on the Right Side of Building Regulations

Building regulations are separate from planning permission. They cover structural stability, fire safety, damp-proofing, ventilation, insulation, electrical safety, and glazing. You can use your local authority building control team or an approved inspector. Work must be inspected at key stages – foundations, damp-proof course, structural openings, and drainage. At the end, you receive a completion certificate. Without it, you may struggle to sell your home.

• Ensure new glazing meets safety requirements for impact and heat loss.
• Ventilation must be adequate, especially for kitchens and bathrooms.
• Electrical work should be signed off by a registered competent person.
• Keep all certificates and warranties in a safe file.

Keeping Your Project on Track

A realistic timeline is 2 to 4 weeks for planning, 6 to 8 weeks for building regulations approval, and 8 to 16 weeks for construction, depending on size and weather. Order materials early – roof lanterns and bespoke glazing can have long lead times. Talk to your neighbours before work starts, and keep the site tidy. Small delays add up: a missing beam or a late inspection can push you back by days.

Expect the unexpected, from buried drains to a sudden spell of rain. The best way to avoid costly delays is to decide everything in advance, communicate clearly with your builder, and keep a contingency fund untouched. A single-storey extension can transform your home, but only if the groundwork – legal, financial and structural – is done properly from the start.