Advice Before Building a House Extension in the UK

How much does a house extension cost in the UK?

House extension construction costs in the UK start at around £3,500 per square metre plus VAT in 2026, and rise significantly depending on specification, location, and site conditions. A modest single-storey rear extension of 20 square metres would cost a minimum of £70,000 in construction alone, before architect fees, structural engineering, and VAT. More complex or highly specified projects will cost considerably more. I cover this in more detail in my articles on house extension costs and in my online consultations.

​

Do I need planning permission for a house extension in the UK?

Not always. Many single-storey extensions can be built under Permitted Development — a set of rights that allow modest work without a full planning application. The rules depend on the size of the extension, how much of your garden it will cover, the type of house you own, and whether you are in a conservation area or have a listed building. If you are in any doubt, speak to an architect before starting. Using Permitted Development incorrectly can create serious legal problems when you come to sell.

​

Do I need an architect for a house extension?

There is no legal requirement to use an architect for a house extension in the UK. However, the homeowners who get the best results — and avoid the most costly mistakes — are those who involve an architect early. An architect helps you make the most of your budget, design something that adds real value to your home, and navigate planning and building regulations. The question is not whether you can afford an architect. It is whether you can afford not to have one.

​

How long does a house extension take from start to finish?

The full process — from first appointing an architect to moving back in after construction — typically takes between 12 and 24 months for a standard single-storey house extension. This includes the design stage (around 2 to 3 months), planning permission if required (8 weeks minimum, often longer), building regulations approval, finding a builder, and the construction itself (typically 3 to 6 months). Projects in conservation areas or involving listed buildings take longer. The biggest delays are usually in finding a reputable builder — the best ones book up months in advance.

​

What is Permitted Development and does it apply to my extension?

Permitted Development is a set of planning rights that allow homeowners to carry out certain types of work without submitting a planning application. For single-storey rear extensions, Permitted Development generally allows extensions up to 4 metres deep on a detached house, or 3 metres deep on a semi-detached or terraced house, subject to height and coverage limits. Larger extensions may be possible under a process called Prior Approval. Permitted Development rights do not apply to listed buildings, some flats, or properties in Article 4 areas. A quick check with an architect before you start will confirm what applies to your home.

​

What is the difference between planning permission and building regulations?

They are two entirely separate approval processes, and you will almost certainly need both for a house extension. Planning permission is about whether you are allowed to build — it considers the design, scale, and impact on the street and neighbours. Building regulations are about whether what you build is safe — they cover structure, insulation, fire safety, drainage, and energy efficiency. Getting planning permission does not mean you comply with building regulations, and vice versa. An architect manages both processes and makes sure neither is overlooked.

​

What should I do first when planning a house extension?

Get independent professional advice before you commit any money. Speak to an architect before you speak to a builder. A builder will tell you what they can build. An architect will tell you what is actually worth building — and what it will realistically cost. My online consultations are designed for exactly this moment: before you have committed to anything, when good advice is cheapest and most valuable.

​

Do I need a structural engineer for a house extension?

Yes, in almost every case. A structural engineer calculates the foundations, beams, and steelwork required for your extension. Their calculations are required by Building Control before construction can begin. Your architect will typically coordinate with a structural engineer on your behalf and incorporate their work into the construction drawings.