How to plan a home extension from scratch

Most home extensions go wrong long before anyone lays a brick. They go wrong at the planning stage — an unrealistic budget, a design that was never going to get permission, or a start date agreed before the drawings were finished. This guide sets out the order things should happen in.

Start with how you want to live, not how many metres you want

The most useful thing you can do first is write down how you actually want to use the space. "We want the kitchen to face the garden and we want to be able to see the children while cooking" is a far more useful brief than "we want a four-metre extension".

Almost every extension we are asked to price could be built at several different sizes. The right one is the size that solves the problem. A well-designed three-metre extension that reconfigures the existing ground floor often works better than a five-metre one that leaves an awkward dark middle to the house and swallows half the garden.

Set a realistic budget, including the parts people forget

Work out your total available budget and then subtract the things that are not construction:

• Design and drawings
• Structural engineer's calculations
• Planning application fees
• Building control fees
• Party Wall surveyor's fees, if a boundary is affected
• Kitchens, bathrooms, flooring and decoration, unless explicitly included in the build quote
• VAT, if quotations are given exclusive of it
• A contingency of around ten per cent for what nobody can see until the floor comes up

That last one matters. Drainage that is not where the drawings suggest, inadequate existing foundations and unexpected ground conditions are the three most common causes of a variation. A contingency turns those from a crisis into an adjustment.

Establish what you can build before you design what you want

This is the step most often skipped. Before committing to a design, the following need checking against your specific address:

• Whether permitted development rights apply, and whether previous extensions have used the allowance up
• Whether the property is in a conservation area or subject to an Article 4 direction
• Whether the property is listed, which requires separate Listed Building Consent
• Whether the plot is in the Green Belt or a National Landscape, where the increase over the original dwelling is limited
• Where the existing drainage runs, and whether a public sewer crosses the site

Any one of these can change what is achievable. Finding out after you have fallen in love with a design is expensive.

Permitted development or a full application

Many single-storey rear extensions can be built under permitted development rights without a planning application, provided they stay within limits on depth, height, materials and coverage. Even when no application is required, it is worth obtaining a Lawful Development Certificate — written confirmation from the council that the work was lawful. A buyer's solicitor will ask for it eventually.

Where a full householder planning application is needed, allow around eight weeks for a decision, plus the time to prepare drawings beforehand. Applications are sometimes refused and resubmitted, so do not treat the eight weeks.

Drawings, structure and a quote you can actually compare

You will need measured drawings of the existing building and the proposal, and structural calculations for any steelwork or foundation design. Only once these exist can builders price the same job.

Be careful comparing quotations that are not based on the same information. A quote produced from a sketch will always look cheaper than one produced from a full drawing package, because it is not pricing the same scope. Ask for a written breakdown, confirm what is excluded, and confirm the specification for the items where cost varies enormously — glazing, kitchens, tiling and heating.

Building Regulations and party walls

Every extension needs Building Regulations approval, separately from planning permission. Inspections happen at defined stages — foundations, damp proof course, structure, insulation, drainage and completion — and the final certificate is what you hand over when you sell.

If the extension is on or within a few metres of a shared boundary, the Party Wall etc. Act 1996 applies. Notices must be served on the affected neighbours, and there are statutory notice periods of one or two months depending on the work. Serve them early. Party wall notices delay far more projects than party wall disputes do.

A realistic timeline

For a typical single-storey rear extension:

• Survey, design and drawings: three to six weeks
• Planning decision, if required: eight weeks or more
• Structural calculations and building control submission: two to three weeks, often overlapping
• Party wall notices: one to two months, run in parallel
• Construction: twelve to twenty weeks depending on size and specification

From first conversation to finished room, six to nine months is normal. Anyone promising materially less than that has usually left something out.

Living through the build

Decide in advance whether you will stay in the property. Most people do. If you are, agree with your builder where the site boundary is, how the rest of the house will be protected, when the kitchen will be out of use and how long for, and where materials and waste will go.

What to ask before you appoint anyone

• Can I see a project you finished more than two years ago?
• Who will be my single point of contact?
• Who is coordinating the trades?
• What is your written payment schedule tied to?
• What insurance do you hold?
• What happens, in writing, if something unforeseen is found?

If you are considering an extension, arrange a free site visit — we will give you an honest view of what your property can take before any design work starts. You can also see our home extensions service or read what to expect from a property refurbishment.