Home extension advice: 8 practical tips for a successful extension

Extensions · Advice

Home extension advice from our architects: the checks to make before you design, the consents an extension can need, how to tender to builders and what to put in the contract.

Last updated: 2 November 2026 · Reviewed by Graham Sproul RIBA, Design & Operations Director

The figures here are approximate and meant to give you an idea, not a quote. Every house and brief is different, so your costs and programme will depend on your project.

How to plan a successful home extension: 8 practical tips

These are the tips we’d give a friend about to extend. They follow the order the work happens in, from the first survey to the contract with your builder. Costs and timings have their own guides, which we link to as we go: the house extension cost guide covers prices per square metre, and our house extension timeline covers how long each stage takes.

1. Start with a measured survey and a desktop study

Everything else is drawn from the survey, so it comes first. A measured building survey gives your architect accurate plans, elevations, sections and levels of the house as it is. Before any design work, we also carry out a desktop study of the site’s constraints and the local planning rules for your house, such as whether it is listed or in a conservation area and whether earlier permissions carry conditions.

2. Settle the planning route early

Some extensions can be built under permitted development; others need a householder planning application. Our guide to planning permission vs permitted development explains the difference. Where the planning decision is the slow part, there are two ways to keep a project moving. On simpler schemes we can start the Building Regulations drawings while planning is pending, at the client’s risk, and we sometimes run a lawful development certificate for a permitted development scheme in parallel with a full application for a larger one.

3. Find the drains before you design

Drains often decide where an extension can go. In Thames Water’s area, building within 3 metres of a public sewer, or within 1 metre of a public lateral drain, usually needs a build-over agreement before work starts, and the charge depends on the pipe size. A CCTV drainage survey, with a camera run through the pipes, shows where the pipes run and what state they are in, which is why it sits on our list of surveys at the Building Regulations stage. Our page on the Thames Water build-over agreement explains the process.

4. Check the trees

Trees affect where foundations can go, and some need the council’s say-so before anyone touches them. A tree with a tree preservation order needs the council’s consent for works. In a conservation area, you must give the council six weeks’ notice before cutting down or pruning most trees: section 211 of the Town and Country Planning Act 1990 lets the work go ahead “after the expiry of the period of six weeks from the date of the notice” unless the council acts first. Check before you design, because a protected tree can rule out a layout.

5. Plan for the neighbours

Party wall notices go to each neighbour, and our fee proposals suggest allowing about £1,000 per neighbour; our party wall guide explains when you need them.

6. Treat Building Regulations as a design stage

Planning drawings show what the extension looks like. Building Regulations drawings show how it is built: the construction specification, foundation and drainage details coordinated with the structural engineer, thermal requirements and typical construction details. With a full plans application, building control must decide “within five weeks or, if you agree, a maximum of two months” (Planning Portal). Your builder prices from these drawings, and anything left open at this stage tends to come back as a variation on site.

7. Tender the full package and compare like for like

We send the design package to up to four vetted builders for tender. When the prices come back, we review each quote, put them side by side in a comparison spreadsheet, raise and answer the builders’ questions, and negotiate to an agreed contract value. A quote that looks cheap is often missing something, and the comparison is where that shows up. On one basement tender we reviewed in September 2026, the most common variation was a waterproofing system specified without the sump, pumps, alarm and discharge route it needs to work.

8. Sign a proper contract and check the insurance

Our usual building contract is the JCT Minor Works Building Contract with contractor’s design, now in its 2024 edition, with JCT Intermediate on larger jobs. It sets the price, the programme, how changes are valued and how payments are certified. We ask contractors for public liability cover of at least £5 million, the standard across our recent JCT contracts. Who insures the existing house and the new work differs from job to job, so read the contract and your own policy together; our house extension insurance guide goes through the options.

The consents and agreements an extension can need

Not every extension needs all of these, but rule each one in or out at the start. The table gives the current charges where they are set nationally or by the water company.

Consent or agreement	When you need it	Charge	Time to allow
Householder planning permission	When the extension isn’t permitted development	£548 from 1 April 2026; due to change around 8 December 2026 (proposed, subject to Parliament)	We advise clients to allow about 3 months
Lawful development certificate	To confirm a scheme is permitted development	£274	Run alongside a planning application if useful
Building Regulations approval	Almost every extension	Set by the building control body	Full plans: 5 weeks, or up to 2 months by agreement
Party wall notices and awards	Work to a shared wall, on the boundary or near a neighbour’s foundations	Allow about £1,000 per neighbour	At least 2 months
Thames Water build-over agreement	Building within 3m of a public sewer or 1m of a public lateral drain	£130 (sewer 175mm or less), £490 (176 to 350mm), £585 plus legal costs from £1,000 (over 350mm)	Before work over the pipe starts
Tree works notice or consent	Trees in a conservation area, or with a tree preservation order	No fee	Six weeks’ notice in a conservation area

Our page on the home renovation process describes each stage, and what we do in it, in more detail.

Worked example: third-party costs for a mid-terrace extension

Take a single-storey rear extension on a mid-terrace house in Thames Water’s area, with a 150mm public sewer running under the back garden and two neighbours who both need party wall notices. These are the charges that sit outside the build cost and your architect’s fees.

Item	Basis	Arithmetic	Allowance
Householder planning application	National fee from 1 April 2026	1 × £548	£548
Party wall, two neighbours	About £1,000 per neighbour	2 × £1,000	£2,000
Build-over agreement	Thames Water Class 1, sewer 175mm or less	1 × £130	£130
Total		£548 + £2,000 + £130 = £2,678	£2,678

Building control charges and the fees for the engineer and surveys come on top, and are quoted for each job. Thames Water reviews its charges every April, and the £548 planning fee is due to change around 8 December 2026 under a proposed new national fee schedule, subject to Parliament, which would take it to about £575, so check both before you budget.

A custom house extension, designed for your house

Off-the-peg extension designs assume a standard house on a standard plot. A custom house extension starts instead from your survey and your brief. We draw up to three concepts in sketch form, using precedent images to test ideas, so you can weigh up layout and light against your budget before committing to one set of planning drawings.

What to compare when the builders’ quotes come back

Quotes for the same drawings can differ a lot, so line up what is in each one before you look at the bottom line.

Check	Why it matters
Is every item in the drawings and specification priced?	Missing items come back later as variations, at a price you can’t compare
What is a provisional sum, and how big is it?	Provisional sums are guesses for work not yet designed; large ones make a quote look cheaper than it is
What is excluded?	Exclusions such as drainage, kitchens, flooring or decoration change the real cost
Programme and start date	A low price with a long programme can cost you more in rent or disruption
Insurance	Ask for the public liability certificate; we look for at least £5 million

FAQs

More house extension advice on our blog

Our extension blog covers the choices that come up on London houses. If you’re weighing up the shape of the extension, read side return vs wraparound extensions.

Sources