Planning your extension

Extensions are priced per square metre until they are not: the kitchen, the glazing and the groundworks are what move the number. Start from a rate, then add the things a rate never includes.

• Extension Cost Per m²: What You'll Really Pay 7 min
• Extension Cost Per m² by Type 6 min
• Does an Extension Add Value to Your Home? 7 min
• Extension vs Loft Conversion vs Moving House 8 min

Do it Cost indexEstimate builder

Before the next stageA cost per square metre you believe, and an all-in figure with the extras in it.

Before a drawing is worth paying for, find out whether the work falls under permitted development or needs a planning application, and whether anything on your postcode takes those rights away. A conservation area, an Article 4 direction, a listed building or a flat rather than a house all change the answer.

• Permitted Development Rights Explained (2026) 10 min
• Do You Need Planning Permission? A UK Homeowner's Guide 8 min
• How Big Can I Build Without Planning Permission? 7 min
• Conservation Area Rules for Homeowners 11 min

Do it Area checker

Before the next stageYou know which route you are on: permitted development, or a planning application.

Permitted development is not a permission, it is the absence of one, and nobody has confirmed anything. A lawful development certificate is the council saying in writing that what you propose is lawful. It costs a fraction of a planning application and it is what a buyer's solicitor asks for years later.

• Lawful Development Certificates Explained 8 min

Before the next stageEither a lawful development certificate applied for, or a planning application submitted.

Architect, architectural technologist or draughtsman, depending on how much design you need as opposed to accurate drawings. You need two sets in the end: planning drawings to get permission, and building regulations drawings that a builder can price and build from. They are not the same, and being quoted from planning drawings is how prices move later.

• Do I Need an Architect? Architect vs Designer vs Draughtsman 7 min

Before the next stageDrawings good enough to apply with, and a clear idea of who is producing the building regulations set.

If anything is being removed, opened up or carried, a structural engineer sizes the beams and specifies the padstones and foundations. Do this before you go out to quote, not after. A builder pricing without calculations is guessing, and the guess is always corrected upwards once the steel schedule appears.

• Do I Need a Structural Engineer? What They Do & When 8 min
• Structural Engineer Costs in 2026: What to Expect 6 min

Before the next stageStructural calculations and a steel schedule, so every builder prices the same thing.

Foundations, drainage, insulation and structural openings are all inspected. Book the inspections into the programme rather than treating them as interruptions to it.

• Extension Building Regulations: Homeowner Guide 10 min
• Building Control: What It Is & When You Need It 10 min
• Building Control Inspections Explained 6 min

Before the next stageAn application in, and the inspection stages in the programme.

Extension foundations usually trigger section 6 because they are dug within three metres of the neighbour and deeper than theirs. If you are also building at the boundary, that is a second notice under section 1. Both are a month, and they run from receipt.

• Party Wall Surveyors: Role, Cost & When You Need One 6 min

Do it Party wall checkParty wall notice template

Before the next stageNotices served, and consent or an award in hand before the digger arrives.

Build the budget from measured figures rather than a hope, and add a contingency you genuinely intend to spend. Check the VAT position while you are here: most work is 20%, but an empty home, a conversion or energy saving materials are not, and on a whole house that is thousands.

• How to Budget and Project-Manage a Home Renovation 9 min

Do it Cost indexEstimate builderVAT checker

Before the next stageA cost range you believe, a contingency, and the VAT rate settled.

Three quotes against the same drawings, the same specification and the same list of what is excluded. Quotes that price different scopes are not comparable however neat the totals look, and the cheapest is usually the one that left something out.

• How to Get & Compare Building Quotes 6 min
• How to Find & Vet a Good Builder 6 min
• Cowboy Builders: Warning Signs Before & During a Job 3 min
• Are Quotes and Estimates Legally Binding? UK Rules 4 min

Do it Quote comparisonSupplier shortlist

Before the next stageAt least three like-for-like quotes, with exclusions in writing.

A contract, a payment schedule tied to stages of work rather than dates, a retention held back, and a start date. A deposit should cover materials the builder has to buy up front, not their cash flow. Everything agreed in the kitchen and never written down is the thing that is argued about in month four.

• Builder Contracts & JCT: Protecting Yourself 8 min
• Builder Deposits: How Much Is Normal in the UK? 4 min
• Contractor vs Individual Trades: Which Should You Hire? 7 min

Do it Build plan and stage payments

Before the next stageA signed contract, a stage payment schedule, and a start date.

Scaffold, skip, where the materials land, whether the pavement or a parking bay is affected, and a licence from the council for any of it that sits on the highway. Tell the neighbours before the first lorry rather than after. It costs an evening and it prevents most of the complaints that stop a job.

• Scaffolding Costs Explained 8 min
• Keeping Neighbours Onside During Building Work 4 min

Do it Letter to the neighbours

Before the next stageScaffold and skip booked, licences applied for, neighbours told.

Keep a site diary, take photographs at every stage that will be covered up, pay against work actually done rather than time elapsed, and put variations in writing before they happen rather than arguing about them after.

• Communicating With Your Builder: Systems That Work 3 min
• Living Through a Renovation 8 min
• Renovation Order of Works 7 min
• Builder Running Late? Delays & Liquidated Damages UK 3 min
• Can I Withhold Payment From My Builder? UK Rules 4 min

Do it Budget tracker

Before the next stageA written record, and payments that match the work on the ground.

Practical completion is not the end. Agree the snagging list in writing, hold the retention until the items are genuinely done, get the completion certificate from building control, and collect the guarantees and certificates. The certificate is the piece of paper a buyer's solicitor will ask for.

• Snagging Lists & Retention: Getting the Job Finished 8 min
• Completion Certificates & Why They Matter 7 min
• Insurance-Backed Guarantees for Building Work Explained 3 min

Do it Snagging listRetention releaseDefective work letter

Before the next stageSnagging closed, completion certificate issued, retention released, paperwork filed.