Trades master copy
Last reviewed: March 2026
What Part A is
Part A is the structural safety bit of Building Regulations. It's about making sure the loads from the building - its own weight, people, furniture, wind, snow, etc. - get safely down to the ground without anything cracking, sagging or collapsing.
For houses and small jobs, it covers foundations, walls, floors, roofs, chimneys and how they all tie together. When you start cutting openings, removing walls or altering roofs, you're in Part A territory.
This guide is a summary to make Part A easier to use on site. It does NOT replace Approved Document A: Structure (latest edition).
You must read and follow the full Approved Document A and any structural engineer's design for your project.
This guide is written for England. Wales, Scotland and Northern Ireland have their own versions of building regulations - the principles are similar but the documents and approval routes differ, so check local requirements if you're working outside England.
Where it applies on your jobs
For most small builders, Part A bites hardest on:
- Knock-throughs and wall removals - opening up between rooms, kitchen-diners, back of house into an extension.
- New openings in external walls - bigger windows, bifolds, French doors.
- Loft conversions - new floor joists, trimming around stairs, cutting into rafters, dormers.
- Extensions - foundations, wall stability, roof/floor tie-ins to the existing house.
- Chimney removals - partial or full, plus how you support what's left.
- Structural repairs - underpinning, crack stitching, lintel replacement.
Part A is also why Building Control care about disproportionate collapse - i.e. one failed bit shouldn't bring the rest of the building down - but on small domestic jobs that mostly boils down to "don't butcher the structure without a proper design".
Key "trigger points" - when you need proper structural design
These are the situations where you should assume you need a structural engineer's calcs and/or Building Control approval, not just "builder's instinct".
Removing or altering load-bearing walls
If a wall is carrying joists, beams, floors, roof, or another wall above, it's load-bearing. Removing it needs a new beam/lintel and proper design.
Signs a wall is likely load-bearing:
- Joists or rafters sit on it or are trimmed into it.
- It's below another wall in the same line.
- It's thick, central, or built in structural materials, not just stud.
Approved Document A won't size your beam - that's what the engineer is for. It just sets the rules the design has to pass.
New openings in external walls (windows, doors, bifolds)
- Enlarging openings in external walls usually needs a lintel or steel designed for the loads above (brickwork, floors, roof).
- Longer openings (combined door + window in a wide knock-through) often need steel beams with bearings and padstones, not just off-the-shelf cavity lintels.
- Both engineer and Building Control will want the lintel/beam spec, bearing lengths, and proof the wall still works in wind load.
Loft conversions and roof changes
- Cutting rafters for rooflights or dormers needs proper trimming and sometimes extra beams.
- New floor joists for a habitable loft are usually deeper and stronger than old ceiling joists, and often need supports at ends and over openings.
- Removing collar ties, purlins or struts without replacement support is a classic way to wreck the roof structure.
Extensions and foundations
- Foundations must be sized to take the loads of the new structure and local ground conditions (shrinkable clay, trees, old drains, nearby buildings).
- Tie-ins to the existing house matter - new work mustn't undermine or overload old foundations and walls.
- For simple single-storey extensions, you can sometimes follow the standard details in Approved Document A; anything unusual needs an engineer.
Chimneys and structural nibs
- Removing a chimney breast at ground/first floor while leaving stack above needs proper support (gallows brackets or steels) and often an engineer's design.
- Removing "small" nibs of masonry can still affect lateral stability or the bearing area under a beam/lintel - don't cut them off without checking what they're doing.
Quick reference table - common jobs
Knock through between kitchen and dining room
- If the wall is load-bearing, you'll need a beam/lintel sized by a structural engineer (or using an agreed standard detail) and Building Control must be notified.
- Temporary propping sequence matters; you can't just hammer out bricks and hope.
- Padstones/bearings at each end of the beam must be sized and built correctly.
Fit 3-4 m wide bifolds in rear wall
- Needs a lintel/steel designed for the span and loads, including any floor/roof above.
- Check you're not removing too much wall that was providing lateral support to the building.
- Building Control will want engineer's calcs and to see the lintel installation (bearings, packing, corrosion protection).
Loft conversion in a 1930s semi
- New floor joists designed for habitable loading, supported correctly at ends and around stair opening.
- Any rafters cut for rooflights/dormers must be properly trimmed and, if necessary, supported by new beams/purlins.
- Don't remove collar ties or load-bearing internal walls below without a designed alternative.
Single-storey rear extension on clay with trees nearby
- Foundations depth and type must take account of soil shrink/swell and tree roots - may need deeper trenches or different solutions.
- Walls, roofs and openings must still meet stability rules in AD A (wall thickness, buttressing, lateral restraint).
- If in doubt, get an engineer to look at the ground and overall stability.
Removing chimney breast at ground floor only
- Remaining chimney above must be supported - usually via steel beams or brackets sized by an engineer, not "a bit of angle iron".
- Load path from the remaining stack to the ground must still be sound.
- Building Control will expect calcs and visible support at inspection.
Routes to compliance for trades
There's no "Part A scheme" - the usual route is:
Follow Approved Document A for "standard" houses
AD A gives simple design rules for traditional masonry/timber houses up to certain sizes/conditions - wall thicknesses, lateral support, small beam/lintel situations.
If your job fits the scope of those examples, you can often follow them without detailed calcs.
Use a structural engineer whenever you're outside the simple cases
New openings in load-bearing walls, major knock-throughs, loft conversions, chimneys, unusual ground, big spans - get calcs.
Building Control will usually insist on engineer's designs and certificates for beams, complex foundations and significant alterations.
Always involve Building Control for structural alterations
Any structural alteration (walls removed, beams added, big openings) needs Building Control approval - either building notice or full plans.
Trying to dodge this bites later at sale: surveyors and buyers hate undocumented structural work.
Check the knock-on into other Parts
Changing structure often affects fire (Part B), sound (Part E), damp (Part C) and sometimes access (Part M).
Don't treat structure in isolation: e.g. opening up around stairs hits Part B, cutting into separating walls hits Part E.
Who is responsible for what
On a typical domestic project:
- The structural engineer (or competent designer) is responsible for sizing beams, lintels, joists and foundations, and for providing calcs and details that comply with Part A.
- The builder/main contractor is responsible for following that design: correct steels, bearings, padstones, propping, and not hacking off extra masonry because "it's in the way".
- The bricklayer/carpenter/roofer is responsible for building the elements (walls, floors, roofs) to the specified thicknesses, ties, straps and bracing.
- The client/owner is responsible for making sure the work goes through Building Control and that they have evidence of compliance when they come to sell.
Blunt version for smaller firms:
If you knock out a wall or cut a roof without calcs or Building Control and something moves or cracks, that's on you. Even if it doesn't actually fall down, you've created a headache when the client sells and the surveyor starts asking for structural paperwork.
Simple rule to drum into your team
If you're removing or cutting anything that's holding something else up - walls, chimney, joists, rafters, nibs - treat it as a Part A job. Don't touch it until there's a clear design (usually from an engineer) and Building Control is in the loop.
On-site checklist (Part A)
Before you start
- Identify all walls and elements you're touching - are they load-bearing, tying in floors/roofs, or supporting other walls?
- Check you have structural drawings and/or calcs for every new opening, beam, loft floor, foundation or chimney alteration.
- Notify Building Control (building notice or full plans) for any structural changes.
While you're working
- Prop correctly before removing masonry or joists - needles, acrows etc., as per engineer's/proper practice.
- Install beams, lintels, joists and foundations exactly to spec - correct size, orientation, bearings, padstones, straps and fixings.
- Don't shave off bearings, nibs or wall thickness to "make it fit" without speaking to the engineer.
When you finish
- Check all structural steel is properly bearing, packed and protected (where required) before you board in.
- Make sure any straps, ties and bracing shown on the drawings are actually fitted.
- Keep copies of calcs, steel schedules and Building Control sign-off with the job record for when the client sells.
Sources
Based on:
- Approved Document A: Structure (current edition) and associated guidance on traditional domestic construction.
- Planning Portal and building-control summaries of Part A requirements for small buildings and structural alterations.
- Industry guidance and examples on knock-throughs, lintels, loft conversions and when to involve an engineer/Building Control.
This guide was last reviewed March 2026. SiteKiln does not provide legal, financial or tax advice. All content is for general information purposes only. Always seek professional advice for your specific situation.
Know someone who needs this?
Working in Wales? The building rules are different. See our Working in Wales guides.
Working in Scotland? Building standards work differently. See our Working in Scotland guides.
Working in Northern Ireland? The system uses Technical Booklets. See our Working in Northern Ireland guides.
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