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Single Storey Rear Extension Price Guide

Location: London (Zones 5–8), North-West London

Typical Market Price (North-West London) – £66,000 (including VAT) - August 2026

This is approximately £3,667/m², which is competitive for North-West London and reflects a realistic price for the scope listed below.

Our Fixed Price - £55,000 (including VAT) (£3,056/m2) – August 2026

Cost Breakdown Price

Materials £15,500

Labour £36,980

Waste removal £2,520

Total £55,000

 

 Typical Property

  • Mid or end terraced house

  • Existing property width: 6.0m

  • Existing property depth: 8.0m

  • Existing floor area: 48m²

  • Garden door: 2m wide

  • Garden window: 1.2m wide

Proposed Extension

  • Rear single-storey extension

  • Size: 6.0m × 3.0m

  • New floor area: 18m²

  • Existing rear wall removed to create an open-plan kitchen/living space.

Scope of Works

Site Preparation

  • Set out extension

  • Site clearance

  • Temporary protection

  • Skip hire and waste management throughout construction

Foundations

  • Excavate foundation trench

    • 600mm wide

    • 900–1000mm deep (subject to Building Control and ground conditions)

  • Dispose of excavated spoil

  • Concrete strip foundations

  • Install foundation blockwork to damp-proof course

  • Damp-proof membrane

Drainage

  • Install new 110mm underground drainage

  • Kitchen waste connection

  • Rainwater gully

  • Inspection chamber modifications (if required)

  • Connect new drainage into existing system

External Walls

  • Construct insulated cavity walls comprising:

  • 100mm facing brick external leaf

  • 90mm Kingspan cavity insulation

  • 100mm Thermalite block internal leaf

  • Wall ties

  • DPC and cavity trays

  • Two air bricks

  • Door and window lintels with cavity tray

  • Internal wall ready for plastering

Structural Works

  • Remove existing rear external wall

  • Retain supporting masonry piers

  • Supply and install structural steel beam

    • 203 × 203 × 46kg UC (or engineer's specification)

  • Padstones

  • Temporary structural support

Ground Floor Construction

  • Sand blinding

  • Damp-proof membrane

  • 100mm reinforced concrete slab

  • 100mm rigid floor insulation

  • Underfloor service conduits

  • 65mm screed

  • Self-levelling compound ready for final floor finish

Roof Construction (Warm Roof)

  • Timber roof joists (150mm)

  • Timber noggins

  • 18mm OSB decking

  • Firring pieces to create roof fall

  • 150mm Kingspan insulation

  • GRP fibreglass waterproof roof system

  • Flashings

  • Fascia and rainwater outlet

Electrical Installation

Installation includes:

  • 8–10 LED downlights

  • 1–2 light switches

  • 4–8 double sockets

  • External double socket

  • External light

  • All wiring

  • Consumer unit connections (where applicable)

  • NICEIC electrical certification

Plumbing & Heating

  • Extend central heating pipework

  • Install one radiator

  • Hot and cold water supplies for kitchen

  • External garden tap

Plastering

  • Plasterboard to all new walls and ceilings

  • Skim coat plaster finish to walls

  • Skim coat plaster finish to ceilings

  • Plastering around structural steel beam

  • Corner beads and trims

  • Making good around electrical boxes and services

  • Smooth finish ready for painting

Internal Decoration

The completed extension will receive a full decorative finish including:

  • Mist coat to all newly plastered walls and ceilings

  • Two coats of quality emulsion paint to walls

  • Two coats of quality emulsion paint to ceilings

  • Primer, undercoat and gloss or satin finish to new timber joinery (where applicable)

  • Final touch-ups following completion of other trades

  • Decoration is carried out using standard white emulsion and white satin or gloss paint unless otherwise agreed.

Doors & Windows

Not included

Bi-fold doors, sliding doors and windows are normally supplied and installed by specialist manufacturers who provide:

  • Manufacturer warranty

  • FENSA certification

  • Installation guarantee

Our team prepares all structural openings ready for installation.

What is Included

✔ Labour

✔ Materials

✔ Waste removal

✔ Building Control inspections

✔ Health & Safety compliance

✔ Site clean upon completion

Price assumptions: Based on good site access, standard ground conditions, no asbestos, no Japanese knotweed, no significant drainage diversions and construction within London Zones 5–8 (North-West London). Any unforeseen structural or ground conditions identified during construction may require additional work.

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