Rear Extensions London: Complete Cost & Design Guide

Rear extensions are the most popular way to add space to London homes. This guide provides detailed information on costs, design, planning, and everything you need to know.

Why Rear Extensions Are So Popular

Rear extensions offer the perfect balance of:

• Reasonable costs (typically £40,000-£70,000)
• Often covered by Permitted Development (no planning needed)
• Significant space gain (15-25m² typical)
• Strong ROI (12-18% property value increase)
• Faster build times (8-14 weeks)

Complete Cost Breakdown

For a Standard 20m² Extension

Structure & Build:

• Groundworks & foundations: £5,000-£8,000
• Brickwork & blockwork: £8,000-£12,000
• Roof structure: £6,000-£10,000
• Structural steel (if required): £3,000-£5,000
• Subtotal: £22,000-£35,000

External Finishes:

• Bi-fold or sliding doors: £3,000-£8,000
• Roof lantern/skylights: £3,000-£7,000
• Brickwork matching: £2,000-£4,000
• Guttering & drainage: £1,000-£2,000
• Subtotal: £9,000-£21,000

Internal Work:

• Plastering: £2,000-£3,500
• Electrics: £2,500-£4,000
• Heating/radiators: £1,500-£2,500
• Flooring (underfloor heating): £2,500-£5,000
• Decoration: £2,000-£4,000
• Subtotal: £10,500-£19,000

Total shell cost: £41,500-£75,000

Add kitchen costs separately: £8,000-£40,000

Permitted Development

The Rules

Single-storey rear extensions can proceed without planning permission if:

• Terraced house: Max 3m extension (6m with neighbor consultation)
• Semi-detached: Max 3m extension (6m with neighbor consultation)
• Detached: Max 4m extension (8m with neighbor consultation)
• Maximum 4m height, 3m to eaves
• Not within 2m of boundary if over 3m high
• Not covering more than 50% of garden

Neighbor Consultation Scheme

To extend 6m (or 8m for detached), you use the “prior approval” process:

• Apply to council for prior approval (£96 fee)
• Council notifies neighbors (21-day consultation)
• If no objections: automatic approval
• If objections: planning officer decides (usually approves)

Design Options

Roof Types

1. Flat Roof with Lantern

Most popular in London. Modern aesthetic, maximum internal height. Add a glass roof lantern for abundant natural light.

Cost: £7,000-£12,000 (roof + lantern)

2. Pitched Roof

Traditional look, better for conservation areas. Provides more ceiling height variation.

Cost: £9,000-£15,000

3. Flat Roof + Skylights

Budget option. Multiple skylights (Velux-style) provide good light at lower cost than lanterns.

Cost: £5,000-£8,000 (roof + 2-3 skylights)

Glazing to Garden

Bi-Fold Doors

Classic choice. Folds away completely to create indoor-outdoor flow.

• 3-panel set (2.7m wide): £2,500-£4,000
• 4-panel set (3.6m wide): £3,500-£6,000
• 5-panel set (4.5m wide): £4,500-£8,000

Sliding Doors

Modern alternative. Large glass panels slide past each other. Less complex than bi-folds, often more reliable.

Cost: £3,000-£7,000

Internal Configuration

Option 1: Full Open Plan

Remove entire rear wall, creating one large kitchen/dining/living space. Requires substantial steel beam (£4,000-£6,000) but creates spectacular entertaining space.

Option 2: Extended Kitchen + Separate Dining

Keep some structure between existing kitchen and new dining area. Creates zoning while maintaining flow.

Option 3: Kitchen + Snug

Extend kitchen into rear addition, use existing kitchen footprint as cozy snug/family room.

Common Challenges & Solutions

Challenge: Limited Natural Light

Solution: Combine roof lantern with large bi-folds. Consider adding internal Crittal screens to “borrow” light from adjacent rooms.

Challenge: Overlooking from Neighbors

Solution: Use obscure glass in side panels above 1.7m height. Plant screening trees/bamboo.

Challenge: Tight Budget

Solution: Build shell first to weatherproof, complete internal finishes in phases. Choose standard rather than bespoke fittings.

Timeline

• Pre-construction (design, Building Control): 6-8 weeks
• Groundworks & foundations: 1-2 weeks
• Structure (walls & roof): 3-4 weeks
• First fix (electrics/plumbing): 1 week
• Plastering & finishes: 2-3 weeks
• Decoration & snagging: 1-2 weeks
• Total: 10-14 weeks