Steel beams and RSJs explained for UK extensions: how engineers choose steel, bearings, padstones, temporary support, Building Control, installation timing and stop-work signs.
Quick answerSteel selection is an engineering decision based on load, span, bearings, wall type, roof/floor loads and deflection limits. A builder should not choose steel by eye. The engineer specifies the beam; Building Control checks compliance; the builder installs with proper temporary support.
DifficultyProfessional onlyTimeRealistic timeline: 1-4+ weeks for engineer survey, calculations, approvals/Building Control coordination, fabrication and delivery. Installation may be 1-3 days for a straightforward opening but can take longer with temporary works, access constraints, multiple beams or complex making-good.DIY costnot a DIY job; homeowner role is planning, documents and checksProfessional costsmall structural openings may run into several thousand pounds once engineer, steel, temporary works, labour, Building Control and making good are included; complex openings or London access can cost much more
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 learnSteel selection is an engineering decision based on load, span, bearings, wall type, roof/floor loads and deflection limits. A builder should not choose steel by eye. The engineer specifies the beam; Building Control checks compliance; the builder installs with proper temporary support.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 requiredStructural drawingsOpening measurementsWall thickness notesTemporary works planBuilding Control checklistMaterials requiredSpecified steel beamPadstonesBolts or plates where designedFire protection boardMaking-good materialsLifting/access equipment
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 limitsStructural openings and beams are Building Regulations matters and should follow structural calculations and approved details.Beam size, bearings, padstones, lateral restraint, fire protection and temporary support should be designed or confirmed by competent professionals.Electrical, plumbing, ceiling and fire-protection details around steels must be coordinated before closing the work.
Price and timeline reality
What affects cost, duration and whether a fixed quote is possible.
Realistic UK cost rangeDIY: not a DIY job; homeowner role is planning, documents and checksProfessional: small structural openings may run into several thousand pounds once engineer, steel, temporary works, labour, Building Control and making good are included; complex openings or London access can cost much morePrices 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
Identify which wall or opening is being changed.
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
Confirm whether the wall is load-bearing with professional advice.
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
Have a structural engineer specify the steel and support details.
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
Plan temporary support before any demolition.
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
Coordinate delivery, lifting, installation and Building Control inspection.
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
Protect, box-in and finish the steel to the required standard.
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.
Structural openings and beams are Building Regulations matters and should follow structural calculations and approved details.
Beam size, bearings, padstones, lateral restraint, fire protection and temporary support should be designed or confirmed by competent professionals.
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 costnot a DIY job; homeowner role is planning, documents and checksProfessional costsmall structural openings may run into several thousand pounds once engineer, steel, temporary works, labour, Building Control and making good are included; complex openings or London access can cost much moreTime requiredRealistic timeline: 1-4+ weeks for engineer survey, calculations, approvals/Building Control coordination, fabrication and delivery. Installation may be 1-3 days for a straightforward opening but can take longer with temporary works, access constraints, multiple beams or complex making-good.Best next stepProperty snagging
Common mistakesCopying a beam size from another house.Removing masonry before temporary support is designed.Forgetting that steel can affect plumbing, electrics, ceilings and finishes.Professional secretsThe steel is only one part of the design; bearings, padstones and temporary works matter too.Access and lifting can be a major cost in tight London homes.Fire protection and documentation should be planned before decorating.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 checklistIdentify which wall or opening is being changed.Confirm whether the wall is load-bearing with professional advice.Have a structural engineer specify the steel and support details.Plan temporary support before any demolition.Coordinate delivery, lifting, installation and Building Control inspection.Protect, box-in and finish the steel to the required standard.Shopping listStructural drawingsOpening measurementsWall thickness notesTemporary works planBuilding Control checklistSpecified steel beamPadstonesBolts or plates where designedFire protection boardMaking-good materialsProfessional 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.
SummarySteel selection is an engineering decision based on load, span, bearings, wall type, roof/floor loads and deflection limits. A builder should not choose steel by eye. The engineer specifies the beam; Building Control checks compliance; the builder installs with proper temporary support.
Keep a simple property log with photos, product names, paint colours, fittings, dates and any professional advice received.
Questions
Frequently asked questions.
Who chooses the steel beam?
A structural engineer should specify it from load, span, structure and bearing conditions. A builder should install to that design, not guess.
How long does steel installation take?
A simple beam may be installed in 1-3 days, but survey, calculations, ordering, Building Control and making-good can make the real timeline several weeks.
When should I stop work?
Stop if a load-bearing wall is being removed without calculations, temporary works, correct bearing details or Building Control coordination.
