Domestic Extension Trade Sequence: From Groundworks to Decoration — Who Does What, When and in What Order

Quick Answer: A domestic extension typically runs across 8–12 trade stages from groundworks to decoration. Each stage has specific handover conditions that must be met before the next trade can start. CDM 2015 applies where there are 2 or more contractors. Building Control inspections are required at key stages (foundations, oversite, drains, structure, insulation, and final completion).

Summary

The trade sequence for a domestic single-storey or two-storey extension is the most common multi-trade construction project in the UK. Getting the sequence wrong costs real money: a plasterer arriving before first-fix electrics and plumbing is complete, or a floor layer turning up before the screed has cured, creates abortive work, damaged materials, and rescheduling delays that can run to weeks.

This guide presents the canonical sequence, dependency rules, and Building Control inspection trigger points. It is designed to be a working reference for the lead contractor, project manager, or tradesperson quoting multi-trade work. Not every project requires every stage — a garage conversion skips foundations, a loft conversion skips drainage. Use the stages relevant to your job and cross-reference linked articles for depth on individual trades.

CDM 2015 (Construction Design and Management Regulations) applies to any domestic project with more than one contractor involved (including subcontractors). The client must be given a F10 pre-construction notification if the project exceeds 500 person-days or 30 working days with >20 workers simultaneously. Even below these thresholds, CDM 2015 still imposes duties — a Construction Phase Plan must be in place before work starts.

Key Facts

• CDM 2015 — applies when 2+ contractors are involved in any domestic project. Client duties apply. Principal Designer and Principal Contractor roles must be assigned.
• Building Regulations Part A — structure; inspections at foundation excavation and pour, and structural frame stages.
• Building Regulations Part C — DPM and oversite slab; inspected before covering.
• Building Regulations Part H — drainage; inspected before backfilling. Drain test (air or water) required.
• Building Regulations Part L — energy; insulation installed correctly, air tightness (SAP calculation), and U-value compliance checked at completion.
• Building Regulations Part P — electrical; notifiable work must be certified by a competent person or Building Control-approved inspector.
• Building Regulations Part G — water; unvented hot water systems require G3 compliance notice.
• Gas Safe — all gas work (new boiler, gas supply to extension) must be carried out by a Gas Safe registered engineer.
• Foundation inspection — Building Control must inspect excavated trenches BEFORE any concrete is poured. Do not pour without sign-off.
• Drainage inspection — drains must be tested and inspected before backfilling. Typically a water test (1.5m head, 30 minutes) or air test to BS EN 1610.
• Practical completion vs. Building Control completion — these are different events. Practical completion is between contractor and client. Building Control completion certificate (or Regularisation Certificate) is a separate document required for building insurance, sale, and mortgage purposes.
• Timber frame / SIPs — if using a prefabricated structural system, delivery programme dictates when ground-floor slab must be ready. This often compresses the groundworks programme.

Quick Reference Table

Detailed Guidance

The Trade Sequence

Stage 0 — Pre-Start (Before Any Groundworks)

Who: Principal Contractor, client, planning authority, building control, utility companies.

Tasks:

• Confirm planning permission and/or permitted development compliance.
• Submit Building Regulations application (full plans or building notice).
• Arrange CDM Construction Phase Plan (required before work starts).
• Carry out site investigation (trial pits if soil type unknown — see soil classification).
• Survey and mark existing services (gas, electric, water, drainage) — call LSBUD or use service trace.
• Notify neighbours if Party Wall Act applies (2 months notice for excavations within 3m of neighbour's foundation; 1 month for party wall work — Party Wall etc. Act 1996).
• Arrange skip permits, scaffolding licences (if over public highway), and any Section 50 licence for crossing the pavement.
• Arrange temporary services (temporary electrical supply, welfare facilities) for sites requiring them under CDM.

Handover to Stage 1: All pre-conditions confirmed. Building Control application registered. CDM plan in place.

Stage 1 — Demolition and Strip-Out

Who: Demolition contractor, builder, groundworker.

Tasks:

• Demolish any existing structure on the extension footprint (conservatories, outbuildings).
• Remove existing paving, garden structures, and vegetation.
• Carry out asbestos survey if any pre-2000 materials present (R&D survey under Control of Asbestos Regulations 2012).
• Strip topsoil and set aside for later reinstatement or disposal. Minimum 150mm topsoil strip over footprint.
• Set up hoarding, dust protection, and access routes as required.

Dependencies: Planning consent confirmed. Asbestos survey complete. Skip in position.

Building Control inspection: Not required at this stage, but notify BCO that work has started (if building notice route).

Stage 2 — Groundworks and Foundations

Who: Groundworker / groundworks contractor.

Tasks:

• Excavate foundation trenches (trench fill or strip, per structural engineer's design).
• CALL BUILDING CONTROL before pouring: BCO must inspect excavated trenches before concrete goes in.
• Pour foundation concrete (typically ST2 or RC30 mix; allow 3–7 days cure before building off).
• Install below-ground drainage runs (including any new foul or surface water drainage). Include rodding eyes and access chambers.
• CALL BUILDING CONTROL to inspect drains before backfilling. Water test or air test required.
• Install DPC-level blockwork to DPC height (typically 150mm above finished external ground level).
• Install DPM (damp proof membrane) under oversite slab — 1200 gauge polyethene minimum, lapped and taped.
• Pour oversite slab (typically ST4 or C25 reinforced, min 100mm, insulation below if floor insulation is to be achieved).
• CALL BUILDING CONTROL to inspect oversite and DPM before covering.
• Backfill and compact around foundations and drainage.

Dependencies: Foundation inspection signed off. Drain test passed. Concrete cured before loading.

See also: strip foundation design, concrete mix designs groundworks, soil classification, underground drainage

Stage 3 — Superstructure: Walls and Roof Structure

Who: Bricklayer / mason, carpenter/joiner, structural steel installer.

Tasks:

• Build cavity walls to plate height (or full height for two-storey). Install wall ties, cavity insulation batts (or closed-cavity), lintels over all openings.
• Install any structural steel beams (RSJ/UC/UB). Structural engineer pad stone details must be followed. Beam installation requires Building Control notification.
• CALL BUILDING CONTROL for structural beam inspection if required by engineer.
• Install roof timbers (rafters, joists, ridge, hips, valleys) or erect roof trusses (if trussed rafter design). Fit roof felt/breathable membrane and battens.
• Install rooflights and Velux openings at rafter stage.
• Install fascias, soffits, bargeboards, and guttering to new roof.
• Install lead flashings, soakers, and step flashings to abutments with existing structure.

Dependencies: Foundation slab cured (minimum 7–14 days). Lintels and beams designed, certified, and on site.

See also: structural steel, party wall, roof trusses, pitched roof structure, leadwork

Stage 4 — Roofing and Weathertight

Who: Roofer, lead worker.

Tasks:

• Tile or slate the new roof (fix tile battens, underlay, and tiles/slates to BS 5534).
• Install all lead flashings, valleys, and abutment details.
• Install flat roof coverings (GRP, EPDM, or felt) if any flat sections.
• Install rooflights and skylights.
• Install guttering and downpipes. Connect to below-ground drainage (or soakaway for surface water — check SUDS requirements under Part H).

Critical milestone: Building is WEATHERTIGHT when this stage is complete. First-fix trades can start.

See also: roof tile types, leadwork, grp fibreglass flat roofing, gutter downpipe sizing

Stage 5 — First Fix: Services

Who: Electrician, plumber, heating engineer. All work concurrently in sequence below.

Order within stage:

1. Electrician (first fix): Run all cabling — ring main, lighting, spur circuits, data. Install back boxes. Mark consumer unit position. Install any distribution boards. Do NOT terminate at this stage.
2. Plumber (first fix): Run all pipework — hot and cold water supply, heating flow and return, underfloor heating manifolds (if wet UFH). Run any waste pipes to drain connections. Do NOT fit valves, taps, radiators, or sanitaryware at this stage.
3. Heating engineer: Install boiler (if new), cylinder, manifolds, and under-floor heating circuits. First-fix pipework including boiler flue position confirmed before wall is plastered.

Key rule: All pipework and cabling must be complete and pressure-tested / continuity-tested before boarding or plastering. Any service routes that will be inaccessible after plastering must be confirmed as leak-free.

Building Control notification: Part P notifiable electrical work must be carried out by a competent person scheme member or notified to Building Control.

See also: first fix second fix, rewire guide, hot water systems, underfloor heating

Stage 6 — Insulation and Airtightness

Who: Insulation installer, builder.

Tasks:

• Install cavity wall insulation (full-fill or partial-fill) if not already done at wall build stage.
• Install roof/ceiling insulation (loft insulation between/over joists, or rigid insulation to warm roof).
• Install floor insulation (rigid PIR boards below screed or between timber joists).
• Install airtightness membrane at ceiling level if required by SAP calculation (common in Part L 2021-compliant extensions).
• Seal all service penetrations with appropriate airtightness tape or grommets.
• CALL BUILDING CONTROL before concealing insulation — BCO may wish to inspect insulation installation.

See also: cavity wall, warm roof cold roof, floor insulation, warm flat roof detail

Stage 7 — Plastering and Dry-Lining

Who: Plasterer, dryliner.

Tasks:

• Erect metal stud or timber stud internal partitions (if not structural).
• Board all walls and ceilings with appropriate plasterboard (standard 12.5mm, or moisture-resistant in wet areas, acoustic where required by Part E).
• Install angle beads, stop beads, and movement beads.
• Apply scratch coat and finish plaster, or tape and joint to skimmed finish.
• Allow minimum 4 weeks drying time (6+ weeks for new concrete block) before decoration. Moisture content must be below 5% before tiling.

Dependencies: First fix complete, checked, and approved. All service routes confirmed. No open pipe joints or cable terminations missing.

See also: skim coat, plasterboard types, metal stud partition installation

Stage 8 — Joinery: First and Second Fix Carpentry

Who: Carpenter/joiner.

Tasks (first fix — some done pre-plaster):

• Fit window frames and door frames (may be done at Stage 3–4 if frames are structural or weather-exclusion critical).
• Install stud walls, noggins for radiators, TV points, socket boxes.

Tasks (second fix — post-plaster):

• Hang all internal doors.
• Fit skirting boards, architraves, window boards.
• Fit loft hatch if applicable.
• Install any built-in joinery (wardrobes, shelving, boxing-in around pipes and beams).

Dependencies: Plaster dry (minimum 4 weeks). Decorating done before skirtings in some programmes, or skirtings done before decoration (trade preference varies).

See also: skirting architrave, door frames, first fix second fix

Stage 9 — Second Fix: Services

Who: Electrician, plumber, heating engineer.

Tasks:

• Electrician: Terminate all wiring, fit switches and sockets, install luminaires, test and certificate all circuits (EICR or new installation certificate). Connect to consumer unit. Notify Building Control if Part P applies.
• Plumber: Fit sanitaryware (if bathroom in extension), connect taps, install radiators, pressurise and commission heating system. Test all water connections.
• Heating engineer: Commission boiler/heat pump, set up controls and thermostats, balance radiators, conduct flue gas analysis, issue Gas Safe certificate (for gas) or MCS commissioning certificate (for heat pump).

See also: testing commissioning, central heating commissioning, radiator balancing

Stage 10 — Tiling and Floor Finishes

Who: Tiler, floor layer.

Tasks:

• Tile wet areas (bathrooms, utility splash zones). Tanking applied before tiling in shower areas and wet rooms.
• Lay floor finishes: screed (allow 28 days drying for traditional sand-cement, 24–48 hours for quick-drying liquid screed); LVT, wood, or tile.
• Install underfloor heating thermostats.

Dependencies: Plaster fully dry, second fix services complete, heating commissioned (so UFH can be tested before floor laid).

See also: large format tiles, screed types, lvt installation, wetroom construction

Stage 11 — Decoration

Who: Decorator.

Tasks:

• Apply mist coat to new plaster (diluted PVA or specific new plaster primer).
• Apply two coats emulsion walls and ceilings.
• Paint woodwork (gloss, satinwood, or eggshell).
• Hang wallpaper (if specified).
• Touch-up after all trades have finished.

Dependencies: Plaster fully dry. Second fix complete. Skirtings and architraves fitted.

See also: interior emulsion, lining paper before decorating

Stage 12 — External Works and Landscaping

Who: Groundworker, landscaper, paving contractor.

Tasks:

• Reinstate external paving (patio, paths, driveway) disturbed during groundworks.
• Lay new external surfaces with SUDS-compliant drainage where required (for driveways to highway, under Part H and GPDO).
• Reinstate landscaping, topsoil, turf, or planting.
• Install external lighting, CCTV, or security lighting circuits (if in scope).

See also: patio laying, suds regulations driveways, drainage landscaping

Stage 13 — Final Sign-Off and Handover

Who: Principal Contractor, Building Control.

Tasks:

• Request final Building Control inspection.
• Provide all certification: electrical installation certificate, Gas Safe certificate, MCS commissioning sheet (heat pump/solar), air test result (if tested), drain test records, structural engineer's completion certificate.
• Issue practical completion certificate to client.
• Provide O&M manual (if commercial or larger project under CDM).
• Register any new boiler, glazing, or appliances for warranty.

Documents to client at handover:

• Building Control Completion Certificate
• Electrical installation certificate (BS 7671 compliant)
• Gas Safe certificate (if gas work done)
• Guarantee/warranty documents
• Instruction manuals for all new appliances and controls

Frequently Asked Questions

In what order do electrician and plumber first fix happen?

Either can go first — there is no absolute rule. In practice, electricians often go first because their cabling is more flexible to route around pipework, whereas pipe routes are less easily diverted. On a well-managed project, both trades work concurrently in different zones. The critical rule: ALL first-fix services must be complete before plastering starts.

When does Building Control need to be notified?

Under the building notice route: at start of work, at foundation excavation, before oversite is covered, before drains are backfilled, at structural frame stage (if applicable), and at final completion. Under full plans approval, the approved plan sets out inspection stages. Always confirm with your local BCO — they vary in their inspection frequency.

Can decoration start before second fix is complete?

Yes — painting can start on walls and ceilings while second fix is underway, as long as sockets, switches, and luminaires are not yet in place. In practice, most programmes decorate after second fix to avoid damage to painted surfaces by the trades.

Who is the Principal Contractor under CDM 2015?

The Principal Contractor (PC) is the contractor in overall control of the construction phase. On a domestic extension where one main contractor manages multiple subcontractors, that main contractor is the PC. The PC must: prepare and maintain the Construction Phase Plan, manage site safety, co-ordinate between trades, and keep a health and safety file.

How long does a typical single-storey extension take?

From groundworks start to handover: 10–18 weeks for a straightforward 20–40m² single-storey extension, depending on size, complexity, and number of trades. The longest variable is plaster drying time and screed curing — these are fixed physical constraints that cannot be accelerated without compromising quality.

Regulations & Standards

• CDM 2015 (Construction Design and Management Regulations 2015) — duty holder roles, Construction Phase Plan, pre-construction notification.

• Building Regulations Approved Document A — structure; foundation and structural frame inspections.

• Building Regulations Approved Document C — site preparation, DPM, and oversite slab compliance.

• Building Regulations Approved Document H — drainage; drain design, testing, and inspection before backfilling.

• Building Regulations Approved Document L (2021) — energy efficiency; insulation U-values and airtightness for extensions.

• Building Regulations Approved Document P — electrical safety; notifiable work requirements.

• Building Regulations Approved Document G — sanitation and hot water; G3 for unvented cylinders.

• Building Regulations Approved Document B — fire safety; fire resistance of structure, means of escape.

• Party Wall etc. Act 1996 — notice requirements for work adjacent to or on shared structures.

• Gas Safe Register — all gas work must be carried out by a Gas Safe registered engineer.

• BS 7671:2018+A2:2022 (18th Edition IET Wiring Regulations) — electrical installation standard.

• Planning Portal: Extensions — planning and permitted development for domestic extensions.

• HSE CDM 2015 guidance — duty holders, Construction Phase Plan requirements.

• NHBC Technical Guidance — construction standards for new-build and extension work.

• Federation of Master Builders — trade sequence guidance and contractor standards.

• soil classification — soil types and foundation depth decisions

• strip foundation design — strip and trench fill foundation design

• building control process — navigating Building Control for extensions

• structural calculations — when and how to commission a structural engineer

• first fix second fix — first and second fix carpentry sequence

• bathroom refit trade sequence — bathroom-specific trade sequence

• cdm regulations — full CDM 2015 duties guide

• building control — Building Control process and inspection stages

• cavity wall — cavity wall insulation types and installation

• rewire guide — electrical first and second fix in detail