Planning to expand your living space? Before choosing between a single or double storey extension, it’s crucial to understand the single vs double storey extension cost breakdown. In this guide, KT5 Construction provides a clear cost comparison, practical insights, and expert advice to help you make the best decision for your home and budget.

As a trusted builder serving London and Surrey, KT5 Construction is committed to helping every homeowner create their dream space with confidence and quality craftsmanship.

What is the Difference Between Single and Double Storey Extensions?

Single Storey Extensions

Single storey extensions add space at ground level and are often used for:

• • Kitchen diners

  • Open-plan living areas

  • Garden rooms

Typical cost in London (2025):
£2,500–£4,000 per m²

Common size: 20–30m²
Estimated total cost: £50,000–£120,000+

Double Storey Extensions

Double storey extensions build both ground and upper floors, adding bedrooms, offices, or bathrooms upstairs.

Typical cost in London (2025):
£130,000–£150,000 total, or £1,500–£1,800 per m²

Savings tip: You only pay once for foundations and the roof, making the cost per square metre lower than two separate single storeys.

Single vs Double Storey Extension Cost Breakdown

Side-by-Side Cost Comparison

What Affects Costs the Most?

• • Size & complexity of the design

  • Quality of materials (standard vs premium)

  • Site constraints (urban London = higher costs)

  • Planning requirements (especially for 2-storey builds)

  • Kitchen/bathroom installation adds £15,000–£40,000.

Planning Permission and Regulations

Single Storey Planning Rules

You may not need full planning permission if under permitted development. Still, a Lawful Development Certificateis highly advised.

Double Storey Planning Rules

Usually requires planning permission unless:

• • Rear extension extends ≤3m

  • Sits ≥7m from rear boundary

  • Matches roof pitch and materials

 Tip: Always consult with KT5 Construction or a planning consultant early in your project.

Pros and Cons of Each Extension Type

Single Storey Pros

• • Lower build cost
  • Faster to complete
  • Less invasive and disruptive
  • Suitable for open-plan living

Double Storey Pros

• • More space for investment
  • Better long-term value
  • Adds multiple functional rooms
  • Cost-effective per square metre

Downsides to Consider

• • Double storeys require more paperwork, planning and longer timelines
  • Higher disruption – may need temporary accommodation
  • Risk of higher contractor, insurance, and surveyor fees

Real Budgeting Advice From KT5 Construction

Full Cost Breakdown (Based on Typical London Builds)

• • Construction: 60% of budget

  • Architect & consultants: 15%–20%

  • VAT: 20% (check for reduced 5% rate eligibility

  • Planning & admin: ~1%

  • Party wall agreements: 3%

  • Insurance: ~1%

  • Fittings (kitchen/bathroom): From £5,000–£40,000+

Hidden Cost Watchlist

• • Drainage and groundwork issues

  • Neighbour disputes (boundary walls)

  • Heating system upgrades

  • Window and door upgrades

How KT5 Construction Helps You Navigate Costs

• • Free initial consultation & site assessment

  • Transparent quoting & itemised cost breakdowns

  • Expert project planning to maximise space

  • Local knowledge of KT postcodes and Surrey planning offices

  • Quantity surveyors available for large projects

  • Focus on cost-efficiency without compromising on quality

FAQs – Homeowners Ask Us

1. Is a two-storey extension better value for money?

Yes. While upfront costs are higher, the cost per m² is often lower and adds more resale value.

2. How long does it take to build each type?

• • Single storey: 3–4 months

  • Double storey: 6–9 months depending on design and weather

3. Will I need to move out during the build?

For single storey – usually not.
For double storey – very likely due to disruption, noise, and structural work.

Final Thoughts on Choosing the Right Extension

When considering a single vs double storey extension cost breakdown, the decision depends on your goals, budget, and tolerance for disruption. If you need a dramatic space upgrade and long-term value, a two-storey extension is a strong choice. If you’re seeking a quick, practical improvement, a single storey may be all you need.

Ready to transform your home?

Contact KT5 Construction for a free consultation and quote tailored to your needs. We specialise in home extensions across Surrey and London, with transparent pricing, trusted tradesmen, and results that speak for themselves.

👉 Book Your Consultation Now

📧 info@kt5construction.co.uk