Double Storey Extensions: Cost, Design & Planning Guide

Double storey extensions provide twice the space for considerably less than twice the cost. This comprehensive guide covers costs, planning, design, and ROI for two-storey home extensions in London.

Why Choose a Double Storey Extension?

Cost Efficiency

The mathematics are compelling. You only pay once for:

• Foundations (the most expensive element)
• Scaffolding
• One roof structure
• Ground floor subfloor and DPM

The second floor costs approximately 60% of the ground floor per square metre, meaning a 40m² two-storey extension might cost £110,000 vs. £150,000 for two separate single-storey projects.

Realistic Costs

Total Project Costs

For a typical 20m² footprint (40m² total over two floors):

• Basic build: £80,000-£100,000
• Mid-spec: £100,000-£135,000
• High-end: £135,000-£180,000+

Breakdown

• Foundations & groundworks: £8,000-£12,000
• Structural frame & walls: £30,000-£45,000
• Roof structure: £10,000-£15,000
• Windows & doors: £10,000-£20,000
• Internal fittings: £15,000-£40,000
• Electrics & plumbing: £8,000-£15,000
• Finishes: £10,000-£25,000

Planning Permission

When You Need It

Two-storey extensions almost always require planning permission. Permitted Development rarely applies because:

• Height restrictions (4m max under PD)
• Proximity to boundaries
• Impact on neighbors (Right to Light)
• Visual impact on street scene

Improving Approval Chances

Key factors councils assess:

• Design quality: Match existing materials and proportions
• Neighbor impact: Demonstrate minimal overshadowing
• Boundary distances: Maintain 1-2m from boundaries where possible
• Overlooking: Avoid windows directly facing neighbors

Design Options

Ground Floor Layouts

Option 1: Kitchen/Dining/Family Room

Open-plan living spanning the full width. Kitchen along one wall, dining table central, family seating area. Typical size: 30-40m².

Option 2: Kitchen + Utility

Larger kitchen with separate utility room tucked behind. Perfect for families needing laundry/storage space.

First Floor Layouts

Option 1: Master Suite

Bedroom (12-15m²) + ensuite (4-6m²) + walk-in wardrobe (4m²). Creates a private sanctuary.

Option 2: Two Bedrooms

Two good-sized bedrooms (10m² + 8m²) sharing a landing. Adds significant family space.

Structural Considerations

Foundations

Two-storey extensions require deeper foundations due to increased load:

• Minimum 1m depth (vs. 750mm for single storey)
• Wider footings (600mm vs. 450mm)
• Possible piling if poor ground conditions (adds £5,000-£15,000)

Steel Beams

Removing walls to create open-plan requires substantial steelwork:

• Ground floor beam: £3,000-£5,000
• First floor beam: £2,000-£4,000
• Structural engineer fees: £1,500-£3,000

Timeline

Realistic timeline for a two-storey extension:

• Planning application: 8-12 weeks
• Building regs & detailed design: 4-6 weeks
• Build phase: 16-20 weeks
• Total: 7-9 months from start to finish

ROI Analysis

Two-storey extensions offer excellent returns in most London areas:

• Adding master suite: 18-25% property value increase
• Adding 2 bedrooms: 20-28% increase (particularly 3-bed to 5-bed)
• Typical spend: £120,000
• Value added (£600k property): £120,000-£150,000
• Net gain: £0-£30,000 (plus lifestyle value)