Notes Extensions, the build in order
How the extension goes together
A single-storey rear extension, built up in the order the work is done on site, from the dig to sign-off. Every stage is the same drawing, with only the parts built so far.
The build, stage by stage
Illustrative. Drawn to true relative scale from a typical 1930s semi-detached house. No size, span, load, depth or level is shown: members are tagged, and an engineer sizes them for your house.
Key to the drawings
- Existing masonry A flat tone: everything already there, cut or seen.
- Brickwork, new Single diagonal hatch. The outer leaf, and new brickwork in elevation.
- Blockwork, new Cross-hatch, so it can never be read as brick beside it.
- Concrete Aggregate triangles and fine dots, scattered at random: foundations, slabs, padstones, lean-mix cavity fill, encasement.
- Undisturbed ground Basket weave, as a band that follows the ground line and fades out.
- Hardcore Large open zigzag, the depth of its own layer.
- Dry pack Fine close stipple, no aggregate: rammed sand and cement over a beam.
- Insulation board The looped line, touching both faces, inside the board outline.
- Timber, cut A box with a cross. Seen along its length, an outline only.
- Steel, cut Solid, at the section's true proportion.
- Pressed steel Lintels, hangers, straps: a line at the wide weight.
- DPC Heavy dashed line, as Approved Document C draws it.
- Membrane, tray Heavy solid line: DPM, cavity tray, roof membrane.
- Load path Where the load goes, from the roof to the ground; dashed where it runs beyond the cut.
- Load marks Arrows at the symbol weight: loads down onto a member, bearing up under a foundation.
- Hidden, beyond or removed Out of the cut, beyond it, or taken out.
- Temporary works, our design Teal line work: needles, props, sole plates, runners, walings, linings. Boxed tags, TW1 and up. Packing is drawn in the dry pack stipple.
- Temporary works, struck Teal, dashed: taken out at this stage.
- By others, beyond Faint: frames, coverings. Not our structure.
- Cloud Confirm on site. Never a revision mark.
- Break line The drawing stops; the building does not.
Illustrative · not for construction
Before the dig
Grey is the house as it stands; the drawings and calculations go to Building Control, and next door gets any Party Wall notice needed. We answer Building Control’s questions on our calculations as part of the job.
Drawing: a section through the back of the house as it stands, in grey, with the extension to come shown as a dashed outline over the garden.
The dig
Building Control is told before work starts, then the topsoil comes off and the trench is dug; Building Control can ask to see it before it is covered. Where its sides need holding up, as here, we design that support and draw it, and the builder puts it in.
Drawing: the topsoil stripped and the trench for the new wall dug, both shown as dashed outlines where the ground was, with the trench sides lined and strutted in teal: our temporary works.
The trench fill
Concrete fills the trench, centred under where the wall will stand, and stops below ground; the trench support comes out as it rises. Its depth is set by the engineer and agreed with Building Control on site, so the drawing breaks it part way down rather than guess.
Drawing: the trench filled with concrete, broken part way down with its base drawn below the break, and the trench support dashed as it comes out.
Up to the damp-proof course
Brick and block leaves rise off the fill, with lean-mix concrete in the cavity stopped short of the damp-proof course. That course runs through both leaves on the same course, clear of the ground, and the trench is backfilled.
Drawing, close up at the foot of the new wall: brick and block leaves on the trench fill, lean-mix concrete in the cavity below the damp-proof course, which runs through both leaves above the ground.
The slab
Hardcore, blinding, a membrane lapped up to the old and new damp-proof courses, then the slab; the old airbricks, beyond this cut, are ducted out to open air. With foundations and slab done, the law treats the work as started, and Building Control is told.
Drawing: the new floor slab on hardcore and blinding, its membrane lapped up to the damp-proof course of the old wall and of the new inner leaf.
Walls, lintel, and plate
The walls rise with insulation in the cavity, closed and damp-proofed under the door sill; a steel lintel spans the opening, with a tray over it to send water out. A timber plate is bedded on the inner leaf and strapped down.
Drawing, closer, the new wall through the door opening: both leaves carried up to the sill with an insulated closer and a damp-proof course under it, the door frame shown faint, a steel cavity lintel over it with a tray, the walls built above it, and a timber plate on the inner leaf.
The beams go in
The wall above and the first floor are held on props we design and draw here, while a pair of beams goes in on padstones and is packed tight. The roof hangs off these beams, so they come first; the old wall below stays.
Drawing, at the old back wall: in teal, a masonry prop head under each leaf of the wall above, one from inside and one from outside, each on a prop to a sole plate, and the first-floor joists on a timber runner and prop; a pair of steel beams, seen end on, packed tight under both leaves. The old wall below still stands.
The props come out
Once the packing has had the time our drawing sets, the builder eases the props down evenly, takes them out in its order, and makes good.
Drawing, at the old back wall: the prop heads, props, runner and sole plates dashed as they come out. The beams now carry the wall above.
The roof structure
Joists hang off the beams on hangers at the house end and sit on the timber plate at the garden end, laid level. Tapered firrings give the fall, and the deck goes on.
Drawing: flat roof joists hung off the beams at the house end and sitting on the plate at the garden end, with firrings and the deck over them.
Weathertight
A vapour control layer, insulation, and membrane go across the deck, and the membrane turns up the house wall under a new cavity tray cut into it. With the fascia on and the doors in, by others, the extension is weathertight.
Drawing, close up where the roof meets the house: the roof insulated and covered, the membrane turned up the old wall under a new cavity tray, and the cover flashing dressed over it.
The knock-through
Only once the extension is weathertight does the old wall below the beams come out, cut down to the top of the slab. This is the order this design assumes.
Drawing, close up at the old back wall: the wall below the beams taken out, shown as a dashed outline, and cut down to the top of the slab. The padstone under the beams shows beyond.
Finished and signed off
The old cavity is closed, then floor insulation and screed run through over the old wall line; the ceiling runs unbroken under the beams, and the steel is fire protected. Building Control is told the work is complete, with your signed statement that, as far as you know, it complies.
Drawing, at the old back wall: its cavity closed at the cut top, the floor insulation and screed run through over it, and the ceiling carried unbroken under the beams.
Where the load goes
The roof loads the joists evenly along their span, and each joist hands its share down at its two ends: into a hanger on the beams at the house end, onto the plate at the garden end. The beams carry it and the wall above to padstones on the piers beyond this cut, dashed, and down to the old footing. The lintel passes its share to the brickwork beside the doors, also beyond the cut, and down to the new foundation.
Drawing: the finished extension with its parts tagged and the load path traced: an even load along the roof joists, a reaction down at each end, into the beams at the house end and down the piers beyond the cut, dashed, to the old footing, and into the lintel at the garden end and down the jamb beyond the cut, dashed, to the new foundation, each ending on the ground under its base.
Sources, all for England, read at legislation.gov.uk on 23 September 2026: the notices before work starts, at a stage Building Control names, and on completion with your signed statement, The Building Regulations 2010, regulation 16; when an extension’s work is treated as started, regulation 46A; notice to next door, the Party Wall etc. Act 1996, sections 2, 3 and 6.