UK house extension first steps: planning permission, permitted development, Building Control, drawings, structural design, realistic timelines, builder checks and common mistakes.
Quick answerA UK house extension does not start with digging. The realistic order is brief, measured survey, planning/permitted-development check, drawings, structural design, Building Control route, party wall/access checks, detailed quote, programme, then site work. Expect weeks or months of preparation before construction.
DifficultyBeginnerTimeRealistic total timeline: 2-6 months before work starts for brief, measured survey, drawings, planning/permitted-development checks, structural input, Building Control route, quotes and contractor booking. Construction time depends on scope.DIY costplanning only; allow money for measured survey, drawings and professional advice before constructionProfessional costearly-stage professional fees often start from a few hundred pounds for advice and can run into several thousand pounds for drawings, structural input and applications before any build cost
Large project
Who this guide is for, and what decision it helps you make.
Best forHomeowners planning extensions, conversions or structural changes who need the correct order before speaking to builders.You will learnA UK house extension does not start with digging. The realistic order is brief, measured survey, planning/permitted-development check, drawings, structural design, Building Control route, party wall/access checks, detailed quote, programme, then site work. Expect weeks or months of preparation before construction.Next actionSend drawings, photos, postcode and your target outcome. Perfect Living can help organise inspection, scope, trade routing and a realistic next step.
DIY or professional?
Can you do this yourself?
The planning, cleaning, preparation and low-risk inspection steps are suitable for careful beginners when the area is safe and accessible.
Before you start
Understand the job before you touch the tools or spend money.
The planning, cleaning, preparation and low-risk inspection steps are suitable for careful beginners when the area is safe and accessible.
Do not DIY if the work involves gas, unsafe electrics, active leaks, structural change, waterproofing failure, tenant disputes, hotel guests, commercial downtime or expensive materials you cannot afford to damage.
Safety warningsStop if you see water near electrics, gas appliances, structural damage or unsafe access.Use gloves, ventilation and eye protection where products or dust are involved.Do not start work if you cannot identify the material, fixing, pipe, cable or surface.UK regulations and professional limitsSome house extensions can be permitted development if strict limits and conditions are met; if limits are exceeded, householder planning permission is likely to be required.Planning Portal guidance says these extension rules apply to houses, not flats/maisonettes, and can be affected by Article 4 directions, conservation areas, listed buildings and previous extensions.Building Regulations approval is separate from planning permission and is normally relevant to structural work, foundations, insulation, ventilation, drainage, electrics and fire safety.
Price and timeline reality
What affects cost, duration and whether a fixed quote is possible.
Realistic UK cost rangeDIY: planning only; allow money for measured survey, drawings and professional advice before constructionProfessional: early-stage professional fees often start from a few hundred pounds for advice and can run into several thousand pounds for drawings, structural input and applications before any build costPrices are guidance only. A fixed quote requires photos, measurements or inspection.What affects the priceSize in m²London access and labour premiumDesign complexityGround conditionsDrainageSteel/structureProfessional feesFinish levelVAT and contingencyWhat affects the timelineSurvey and drawingsPlanning or permitted development checksStructural calculationsBuilding ControlParty wallLead timesWeatherInspectionsSnagging
Step by step
How an experienced tradesperson would think through it.
Step 1
Define the reason for the extension before choosing a design.
Why it matters: this stage reduces mistakes before they become expensive. Work slowly, check the result, and do not move to the next step until the area is safe and understood.
Step 2
Check whether permitted development could apply or whether planning permission is likely.
Why it matters: this stage reduces mistakes before they become expensive. Work slowly, check the result, and do not move to the next step until the area is safe and understood.
Step 3
Measure width, depth, boundaries, drainage, access and existing floor levels.
Why it matters: this stage reduces mistakes before they become expensive. Work slowly, check the result, and do not move to the next step until the area is safe and understood.
Step 4
List structural, plumbing, heating, electrical and drainage changes.
Why it matters: this stage reduces mistakes before they become expensive. Work slowly, check the result, and do not move to the next step until the area is safe and understood.
Step 5
Create a rough budget with professional fees, labour, materials, VAT, waste and contingency.
Why it matters: this stage reduces mistakes before they become expensive. Work slowly, check the result, and do not move to the next step until the area is safe and understood.
Step 6
Speak to the right professionals before buying materials or booking builders.
Why it matters: this stage reduces mistakes before they become expensive. Work slowly, check the result, and do not move to the next step until the area is safe and understood.
Technical note
Construction projects fail when the order is wrong.
Some house extensions can be permitted development if strict limits and conditions are met; if limits are exceeded, householder planning permission is likely to be required.
Planning Portal guidance says these extension rules apply to houses, not flats/maisonettes, and can be affected by Article 4 directions, conservation areas, listed buildings and previous extensions.
Decision flowBuild sequence
Brief, survey, permissions, structural design, Building Control, groundworks, first fix, insulation, second fix, finishes and snagging must stay in order.
Documents, approvals and rules
What to check before the job becomes expensive.
Regulation notesPlanning permission or permitted development should be checked before design is treated as final.A Lawful Development Certificate can be useful where permitted development is relied on.Building Regulations and Building Control are separate from planning and usually matter for structure, insulation, fire safety, drainage and electrics.The Party Wall etc. Act can apply near shared walls, boundaries or excavations close to neighbours.Conservation areas, listed buildings, flats and maisonettes can have stricter rules.Useful documentsPhotos of the existing conditionMeasurements and room/property detailsProduct manuals or model numbers where relevantQuotes, invoices and certificates for previous workDo not rushDo not hide defects before they are diagnosed.Do not buy expensive materials before checking compatibility.Do not accept a vague quote for regulated or hidden work.
Cost guide
What it usually costs.
DIY costplanning only; allow money for measured survey, drawings and professional advice before constructionProfessional costearly-stage professional fees often start from a few hundred pounds for advice and can run into several thousand pounds for drawings, structural input and applications before any build costTime requiredRealistic total timeline: 2-6 months before work starts for brief, measured survey, drawings, planning/permitted-development checks, structural input, Building Control route, quotes and contractor booking. Construction time depends on scope.Best next stepProperty snagging
Common mistakesStarting with finishes instead of planning constraints.Forgetting drains, manholes, boundaries and access.Assuming a neighbour's extension means yours will be approved.Professional secretsA clear brief saves more money than hunting for the cheapest builder.Planning rules are only one part of the job; Building Regulations still matter.A measured sketch with photos makes early advice much more useful.Red flagsA quote with no scope, exclusions or assumptions.Someone pushing you to start before photos, measurements or access are clear.No explanation of what happens if the job changes.A builder willing to remove walls, dig foundations or install steels without drawings, calculations or inspection route.
Before hiring anyone
Questions that protect your budget before work starts.
Ask these questionsWhat exactly is included and excluded from the price?Can this be quoted from photos, or is inspection required?What could change the price once work starts?Who supplies materials, and who carries the risk if they do not fit?What happens if hidden damage is found?Which drawings, calculations, permissions and Building Control stages are needed before work starts?What Perfect Living needsPostcode and access detailsPhotos or short video of the issueMeasurements, product links or drawings where relevantDeadline, tenant/guest constraints and parking notesHow Perfect Living can helpSend drawings, photos, postcode and your target outcome. Perfect Living can help organise inspection, scope, trade routing and a realistic next step.If the job is simple, photos may be enough for a price range or fixed quote.If the job involves hidden defects, structure, water, electrics or high-value finishes, inspection is the safer first step.
Interesting factMost bad DIY results are caused by preparation mistakes, not the final visible step.
Keep a simple property log with photos, product names, paint colours, fittings, dates and any professional advice received.
Troubleshooting
If it does not go to plan.
IssueLikely causeFix
The result looks unevenPoor preparation, wrong product or unsuitable surfaceStop, clean back where safe and reassess the method before continuing.
The job takes much longer than expectedCondition, access or hidden defects were underestimatedBreak the task into sections or request a fixed professional quote.
Something feels unsafeUnknown electrics, water, gas, access or structural riskStop and contact the correct professional.
Printable checklist
Before you start, check these items.
Preparation checklistDefine the reason for the extension before choosing a design.Check whether permitted development could apply or whether planning permission is likely.Measure width, depth, boundaries, drainage, access and existing floor levels.List structural, plumbing, heating, electrical and drainage changes.Create a rough budget with professional fees, labour, materials, VAT, waste and contingency.Speak to the right professionals before buying materials or booking builders.Shopping listTape measureCameraSite notebookExisting floor planBudget spreadsheetLocal authority planning portalPlanning notesMeasured sketchWish listMust-have listProfessional decision pointsStop if you see water near electrics, gas appliances, structural damage or unsafe access.Use gloves, ventilation and eye protection where products or dust are involved.Do not start work if you cannot identify the material, fixing, pipe, cable or surface.
SummaryA UK house extension does not start with digging. The realistic order is brief, measured survey, planning/permitted-development check, drawings, structural design, Building Control route, party wall/access checks, detailed quote, programme, then site work. Expect weeks or months of preparation before construction.
Keep a simple property log with photos, product names, paint colours, fittings, dates and any professional advice received.
Questions
Frequently asked questions.
Can I build an extension in a few days?
No. A small extension normally takes weeks on site after design, permissions, Building Control and contractor scheduling. The full journey commonly takes months.
Do I always need planning permission?
Not always. Some house extensions can fall under permitted development, but the exact property, location, size, previous extensions and restrictions must be checked.
Is Building Control the same as planning?
No. Planning controls whether the development is allowed in planning terms. Building Control checks technical compliance such as structure, fire safety, insulation, drainage and ventilation.
