House Extension Build Process: Planning Timeline, Building Notice vs Full Plans, CDM Duties and Construction Sequence

Quick Answer: A typical single-storey rear extension takes 3–6 months from planning application to completion. Most extensions ≤4m (detached) or ≤3m (semi/terraced) are permitted development. The build sequence is: strip out → foundations → drainage → slab → blockwork/frame → roof → first fix → plaster → second fix → decoration. Building Control must inspect at foundations, damp proof course, floor slab, and completion.

Summary

For most tradespeople, a house extension is either the entire job (if you're a main contractor or builder) or one phase of it. Understanding the full process helps you sequence your own trade correctly, anticipate delays, and give clients realistic timelines.

The biggest timeline killers are: waiting for planning (8–10 weeks), party wall disputes (if semi-detached or terraced), structural calculations delays, and material procurement (steel beams, windows). Extensions that start without confirming structural beam sizes are particularly prone to delay — the steelwork cannot be ordered until the structural engineer issues their calculations.

CDM 2015 applies to all construction work in domestic settings. For single-site, single-client domestic projects, the domestic client duty holder responsibilities are simpler than commercial, but the Principal Designer and Principal Contractor duties still exist where there is more than one contractor.

Key Facts

• Permitted development (England) — Single-storey rear extension: ≤4m from rear wall (detached), ≤3m (other). Under 3m wide: no PD restrictions on the sides. Subject to conditions (height, materials, % land covered). Check GPDO 2015 Class A.
• Prior Approval (Neighbour Consultation Scheme) — Extensions 4–8m detached, 3–6m other, can go through Prior Approval instead of full planning. LPA must be notified; neighbours consulted; decision within 42 days.
• Full planning application — Required for: larger extensions, those in restricted areas (conservation area, AONB), front extensions, extensions affecting walls shared with a listed building.
• Party Wall Act 1996 — Required notice for: work on or near a party wall (1m Notice to adjacent owners), excavations within 3–6m of neighbour's foundation (Section 6 notice). 2-month notice for party wall works; 1 month for Section 6.
• Building Notice vs Full Plans:
  • Building Notice: faster start (notify and start within 2 days); no drawings checked upfront; risk of changes during build
  • Full Plans: drawings submitted and approved (typically 5–8 weeks) before starting; inspections planned; better for complex jobs
• CDM 2015 — For domestic clients: if more than one contractor, appoint a Principal Designer (PD) and Principal Contractor (PC). Pre-construction information must be provided. Construction Phase Plan required. F10 notification if project >500 person-days or >30 working days with >20 workers.
• LABC inspection stages — Foundations (before concrete poured), DPC (before backfill), ground floor slab (before pour), drainage test, final/completion.
• VAT on extensions — Residential extensions: standard rate 20% VAT on labour and materials (no zero rate unless new build or conversion). Converting non-residential to residential: 5% reduced rate.

Quick Reference Table

Detailed Guidance

Planning Stage

Step 1: Determine PD eligibility

• Measure proposed extension from original rear wall (not extended)
• Confirm no PD conditions removed (Article 4 Direction, prior planning conditions)
• Check Land Registry: is the property in a conservation area?
• Contact LPA planning department helpline for informal guidance if unsure

Step 2: Submit if required

• PD: no submission needed, but a Certificate of Lawful Development (CLOPUD) gives legal certainty (useful for sale). Apply to LPA.
• Prior Approval: submit to LPA on form PA4 with site plan
• Full planning: submit online (planningportal.co.uk) with drawings, forms, and fee (£206 for householder application in England)

Step 3: Party Wall Act

• If the proposed works are within 3m of a neighbour's building, notify neighbour under Section 6 (excavations)
• If cutting into or building on a party wall, serve Section 1 or Section 2 notice

Design and Structural Engineering

Before building starts, ensure:

• Structural engineer has designed and issued calculations for: (a) opening between existing house and extension (RSJ/beam), (b) extension roof structure, (c) foundation type based on soil investigation
• Material schedule confirmed: brick/block spec, window spec (FENSA notification required), roof tiles
• Drainage design: new drain falls and connections to existing sewer confirmed. If connecting to public sewer, check if self-laid connection or adoption point requires utility notification.

Avoid the common delay: Order steel beams before work starts. Lead time for structural steelwork: typically 4–6 weeks. Many extensions have stood as completed blockwork waiting for a beam.

Building Control Route: Full Plans vs Building Notice

Full Plans:

• Submit drawings to LABC (or Registered Building Inspector): floor plan, elevations, sections, structural details, drainage, insulation schedule
• Fee: typically £400–£800 for a single-storey extension (LA-dependent)
• Approval typically within 5–8 weeks; conditions may be attached
• Staged inspections booked in advance
• Recommended for complex extensions, any structural complications, where client wants certainty

Building Notice:

• No drawings submitted; notify LABC 2 days before starting
• Lower upfront fee but same inspection process during build
• Risk: LABC inspector may require changes on site that were not anticipated
• Recommended for simple, standard extensions where you know the specs thoroughly

CDM 2015 for Domestic Extensions

Most single domestic extensions are CDM notifiable only if they exceed 500 person-days or have >20 workers for >30 days. For smaller projects:

• Domestic client duties: The homeowner (domestic client) is the client. Their duties are reduced under CDM 2015 — they do not need to appoint a Principal Designer or Principal Contractor if there is only one contractor
• More than one contractor: If more than one contractor works on the site at the same time, the domestic client must appoint a Principal Contractor (typically the main builder). If it would normally require a Principal Designer, the lead designer takes on that role by default unless another party is appointed
• Pre-Construction Information (PCI): Provide to contractors: asbestos surveys, plans, structural information
• Construction Phase Plan (CPP): Required from the Principal Contractor — can be simple for a small extension

Construction Sequence

Week 1–2: Groundworks

1. Mark out extension footprint with pegs and lines
2. Excavate topsoil; store separately for reinstatement
3. Excavate foundation trenches (depth per structural engineer — minimum 450mm in clay soil, typically 600–900mm)
4. Lay drainage runs (set falls before backfilling)
5. LABC inspection of foundation excavations
6. Pour strip foundations or trench-fill concrete to within 150mm of DPC level
7. Complete drainage connection to existing system; LABC drainage inspection

Week 2–4: Masonry

1. Build up cavity walls or solid walls to DPC level
2. LABC DPC inspection
3. Install floor insulation and DPM
4. LABC slab inspection; pour concrete slab
5. Build walls to window/door head height
6. Install lintels; insert window/door frames (or boxes if windows ordered separately)
7. Build walls to full height; install cavity wall ties at 900mm horizontal, 450mm vertical

Week 4–8: Roof

1. Install structural roof members (rafters, ceiling joists, or engineered trusses)
2. Fix sheathing/sarking boards or batten as per roof system
3. Install breathable underlay; batten and counter-batten
4. Fix tile/slate covering; fix ridge tiles (or dry-fix system)
5. Install flashings at abutment walls and any valley

Week 6–10: Windows, Doors, External Finish

1. Install windows and external doors in prepared openings
2. Internal: install steel beam where existing house opens to extension (LABC may inspect padstones and beam bearing)
3. External: render, point, or clad as per design
4. Make good cavity walls and ties at opening to existing house

Week 8–14: First Fix and Finishes

1. Electrician: first fix (cable routes, back boxes)
2. Plumber: first fix (water, heating pipework)
3. Screeding or floor preparation
4. Plasterboard and skim

Week 12–18: Second Fix and Completion

1. Electrician: second fix (sockets, switches, consumer unit)
2. Plumber: second fix (sanitary ware, rad valves, boiler changes if needed)
3. Decorator: painting and decorating
4. Kitchen/joinery: fit skirting, architrave, kitchen if applicable
5. LABC final inspection (all trades complete)
6. EIC, Gas Safe certificate, FENSA certificate as applicable

Frequently Asked Questions

How long does a single-storey extension take to build?

A straightforward single-storey rear kitchen extension (20–25m²) typically takes 10–14 weeks on site from groundworks to completion. Add 8–10 weeks for full planning, plus time for engineering and material procurement. Total project timeline from instruction to key in door: 6–9 months.

Do I need an architect for a simple extension?

Not legally. Building Regulations require drawings showing structural design, insulation, drainage and fire safety, but these can be produced by a building technician, structural engineer, or competent builder. An architect adds design value on complex or visually prominent projects; a technician is more cost-effective for straightforward rear extensions.

What inspections does LABC carry out?

LABC typically inspects at: foundation excavations (before concrete); DPC (before backfilling); ground floor (before slab pour); drainage (water test before backfilling); structural beam installation (if required); and final inspection (when complete). Some LABC offices also inspect at roof-level on larger projects.

Regulations & Standards

• GPDO 2015 Schedule 2 Part 1 — Permitted development rights for householder extensions

• Building Regulations 2010 — All Parts applicable to extensions (A, B, C, E, F, J, K, L, P, Q)

• CDM Regulations 2015 — Construction design and management: domestic client duties

• Party Wall Act 1996 — Notices and party wall awards

• Planning Portal — Householder extensions

• HSE — CDM for small projects

• LABC — Building Notice guidance

• planning permission — Permitted development limits in detail

• party wall — Party Wall Act notices and surveyor process

• structural steel — RSJ beam installation

• foundations — Foundation types and soil suitability