Single-storey extension guide: planning, cost and build stages
Single-storey extension guide for UK homeowners: realistic timelines, planning permission, permitted development, Building Control, foundations, steels, roof, first fix and second fix.
Quick answerA single-storey extension should be built in a controlled sequence: permission or lawful-development check, measured plans, structural design, building control route, groundworks, foundations, damp protection, walls, steel where specified, roof, windows, first fix electrics and plumbing, insulation, plasterboard, plastering, second fix, decoration, snagging and sign-off. If the order is wrong, the project becomes expensive to correct.
DifficultyAdvancedTimeRealistic total timeline: 3-7 months from first idea to completion. Typical preparation: 4-12 weeks for survey, drawings, planning/permitted-development advice, structural design, Building Control and quotes. Typical on-site build: 8-16 weeks for a straightforward single-storey rear extension; longer for drainage changes, steels, kitchens, high-end finishes, poor access or delays.DIY costplanning only; construction is a professional projectProfessional costtypical UK build guidance often sits around £1,800-£3,500 per m² in 2025, with London, poor access, glazing, kitchens, drainage, VAT, fees and finish level pushing costs higher
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 single-storey extension should be built in a controlled sequence: permission or lawful-development check, measured plans, structural design, building control route, groundworks, foundations, damp protection, walls, steel where specified, roof, windows, first fix electrics and plumbing, insulation, plasterboard, plastering, second fix, decoration, snagging and sign-off. If the order is wrong, the project becomes expensive to correct.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.
Tools and materials
What you need before you start.
Required skillsCareful observationBasic preparationFollowing instructions in orderTools requiredMeasured drawingsStructural engineer drawingsBuilding Control inspection planTape measureLaser levelPhoto logVariation logQuote comparison sheetMaterials requiredConcrete and reinforcement where specifiedBlocks or brickworkDamp-proof membrane and damp-proof courseCavity insulationSpecified steel beams and padstonesRoof structure and coveringWindows and external doorsElectrical and plumbing first fix materialsPlasterboard, plaster, primer and paint
Safety and UK regulations
Read this before touching the job.
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 limitsSingle-storey rear extension permitted development limits can depend on house type, original house, previous extensions, height, boundaries, designated land and prior approval for larger rear extensions.Building Regulations approval is normally required for extension work covering foundations, structure, damp protection, insulation, ventilation, drainage, glazing, electrics and fire safety.Party Wall matters, build-over drainage agreements, conservation/listed restrictions and lease/freeholder consent may also apply depending on the property.
Price and timeline reality
What affects cost, duration and whether a fixed quote is possible.
Realistic UK cost rangeDIY: planning only; construction is a professional projectProfessional: typical UK build guidance often sits around £1,800-£3,500 per m² in 2025, with London, poor access, glazing, kitchens, drainage, VAT, fees and finish level pushing costs higherPrices 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
Permission and legal route
Before anyone digs, confirm whether the extension needs householder planning permission, prior approval, permitted development evidence, party wall notices, landlord/freeholder consent or conservation/listed-building input. This is the commercial foundation of the project: without the right route, a technically good build can still become a legal and resale problem.
Step 2
Measured plan and scope
The plan should show existing walls, proposed walls, openings, roof form, drainage, manholes, floor levels, sockets, heating, kitchen/bathroom changes and finishes. A vague sketch creates vague quotes. A proper plan lets each contractor price the same job.
Step 3
Structural design and steel specification
If walls are removed or large openings are created, a structural engineer should specify beam size, bearings, padstones, connections and temporary support. The steel is selected from load, span, wall type, roof load, floor load and bearing conditions, not from guesswork or what worked in another house.
Step 4
Builder quote and programme
A serious quote separates groundworks, foundations, drainage, shell, roof, windows, first fix, plastering, second fix, decorating, waste and contingency. It should say what is excluded and when client decisions are needed.
Step 5
Site setup and protection
Before demolition or digging, protect floors, access routes, neighbour boundaries, gardens and internal rooms. Confirm skip location, waste route, parking, working hours, dust control and where materials will be stored.
Step 6
Excavation and trench inspection
The trench is dug to the depth and width required for the ground conditions and Building Control expectations. Roots, made ground, drains, old foundations and clay can change the design. Building Control often needs to inspect before concrete is poured.
Step 7
Reinforcement and foundation pour
Some foundations require reinforcement, mesh or specific detailing. Concrete should be placed into a clean trench, levelled properly and protected from contamination, freezing, drying too quickly or heavy rain where relevant.
Step 8
Foundation curing and oversite preparation
Concrete gains strength over time. Light setting is not the same as full design strength. The exact waiting time depends on mix, weather, load and engineer/builder guidance, but the project should not rush heavy loading before the foundation is ready. Oversite, hardcore, damp-proof membrane and floor insulation must be sequenced correctly.
Step 9
Brickwork, blockwork and insulation
Walls are built plumb, level and tied correctly to the existing structure. Cavities, insulation, DPC, weep vents, lintels and thermal detailing matter as much as the visible brick. Insulation must be continuous and dry.
Step 10
Steel beam installation and structural openings
Steels are installed only after temporary support is in place and bearings/padstones are ready. Openings must follow the engineer's drawing. The beam may need fire protection and coordination with ceiling levels, services and finishes.
Step 11
Roof structure and weatherproofing
The roof choice depends on design, drainage, neighbours, height limits, cost and internal ceiling goals. Flat roofs need correct falls and drainage. Pitched roofs need correct structure, covering and flashing. Weatherproofing should happen before sensitive internal work.
Step 12
Windows, doors and external envelope
Windows and doors should be ordered to accurate structural openings, installed plumb and sealed correctly. The external envelope must become weather-tight before plasterboard, plaster and decoration are exposed to moisture.
Step 13
First fix electrics, plumbing and heating
First fix is hidden work: cables, back boxes, pipework, waste, heating routes, ventilation and any data/cable routes before walls close. Positions should be marked from the final kitchen, bathroom, lighting and furniture layout.
Step 14
Insulation, plasterboard and plaster
Insulation should match the specification and be installed without gaps, compression or wet areas. Plasterboard type matters in wet, fire, acoustic and high-impact zones. Plastering should happen after first fix inspection and before second fix.
Step 15
Second fix, painting, snagging and handover
Second fix is the visible stage: sockets, switches, taps, radiators, skirting, architraves, doors, handles, sanitaryware, final lighting, painting and silicone. The final stage should include testing, certificates where relevant, warranties, product records and a snag list.
Technical note
Construction projects fail when the order is wrong.
Single-storey rear extension permitted development limits can depend on house type, original house, previous extensions, height, boundaries, designated land and prior approval for larger rear extensions.
Building Regulations approval is normally required for extension work covering foundations, structure, damp protection, insulation, ventilation, drainage, glazing, electrics and fire safety.
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; construction is a professional projectProfessional costtypical UK build guidance often sits around £1,800-£3,500 per m² in 2025, with London, poor access, glazing, kitchens, drainage, VAT, fees and finish level pushing costs higherTime requiredRealistic total timeline: 3-7 months from first idea to completion. Typical preparation: 4-12 weeks for survey, drawings, planning/permitted-development advice, structural design, Building Control and quotes. Typical on-site build: 8-16 weeks for a straightforward single-storey rear extension; longer for drainage changes, steels, kitchens, high-end finishes, poor access or delays.Best next stepKitchen renovation
Common mistakesPouring foundations without the right inspection.Choosing paint and flooring before drainage and structure are solved.Not stopping work when hidden stages look wrong.Paying the final balance before certificates, snagging and handover documents are complete.Professional secretsDo not let visible finishes start until the shell is dry and services are tested.The best time to control quality is before work is hidden.Ask for photos of trench, DPC, insulation, steels, first fix and roof details before they are covered.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 checklistPermission and legal route.Measured plan and scope.Structural design and steel specification.Builder quote and programme.Site setup and protection.Excavation and trench inspection.Shopping listMeasured drawingsStructural engineer drawingsBuilding Control inspection planTape measureLaser levelPhoto logVariation logQuote comparison sheetConcrete and reinforcement where specifiedBlocks or brickworkProfessional 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 single-storey extension should be built in a controlled sequence: permission or lawful-development check, measured plans, structural design, building control route, groundworks, foundations, damp protection, walls, steel where specified, roof, windows, first fix electrics and plumbing, insulation, plasterboard, plastering, second fix, decoration, snagging and sign-off. If the order is wrong, the project becomes expensive to correct.
Keep a simple property log with photos, product names, paint colours, fittings, dates and any professional advice received.
Questions
Frequently asked questions.
How long does a single-storey extension really take?
A straightforward build often takes around 8-16 weeks on site, but the full process from first idea to completion can easily be 3-7 months once design, permissions, Building Control, quotes and lead times are included.
When should foundations be poured?
Only after the trench is dug to the required depth/width and Building Control or the relevant inspector has checked it where inspection is required.
How long should foundation concrete stand?
Concrete gains strength over time. Light construction may continue according to the engineer/builder sequence, but heavy loading should not be rushed. Follow the structural design, concrete specification, weather conditions and Building Control/builder guidance.
When should I stop the builder?
Stop before work is hidden if foundations, DPC, insulation, steels, roof falls, drainage, first fix or waterproofing look wrong, undocumented or different from drawings.
