If you’re researching the cost of a two-storey extension, you’re likely serious about expanding your home.
Whether you need a larger kitchen, an extra bedroom, a home office, or more family space, a two-storey extension is one of the most cost-effective ways to significantly increase square footage without moving.
At Garnet Construction, we build high-quality two-storey extensions across Hertfordshire, Essex, and North London, and one of the first questions we’re asked is:
“How much will it cost?”
Here’s a realistic guide for 2026.
Average Two-Storey Extension Costs (UK – 2026)
While every project is different, here’s a general guide:
Basic Build Cost (Per m²)
- £1,800 – £2,500 per m² (standard specification)
- £2,500 – £3,200+ per m² (higher-end finish)
Typical Total Project Costs
| Extension Size | Estimated Cost Range |
|---|---|
| 20m² per floor (40m² total) | £75,000 – £110,000 |
| 30m² per floor (60m² total) | £100,000 – £160,000 |
| 40m² per floor (80m² total) | £135,000 – £220,000 |
These figures include structure, insulation, roofing, windows, doors, and core construction—but final costs depend on specification and complexity.
Why Two-Storey Extensions Offer Better Value Per m²
A two-storey extension is often more cost-effective than building two separate single-storey extensions because:
- Foundations are shared
- Roofing costs are proportionally lower
- Services (plumbing, electrics) can be integrated efficiently
- Scaffolding and labour overlap
You’re effectively doubling your space without doubling your build cost.
What Affects the Cost of a Two-Storey Extension?
1. Size and Layout
Larger footprints increase material and labour costs, but price per m² may reduce slightly on bigger builds.
2. Specification Level
- Standard finishes vs premium kitchens and bathrooms
- Bi-fold doors vs standard glazing
- Underfloor heating vs radiators
- Roof lanterns or structural steel requirements
3. Structural Complexity
- Steel beams (RSJs)
- Removing load-bearing walls
- Ground conditions requiring reinforced foundations
4. Planning Permission & Regulations
Most two-storey extensions require planning approval. Costs may include:
- Architect drawings
- Structural engineer calculations
- Planning application fees
- Building control inspections
5. Location
Build costs in Hertfordshire and Essex tend to be slightly above national averages due to labour and demand.
What’s Usually Included in a Two-Storey Extension Build?
With Garnet Construction, a full-service build typically includes:
- Groundworks and foundations
- Brickwork and structural frame
- Roofing and insulation
- Windows and external doors
- Internal walls and plastering
- First and second fix electrics and plumbing
- Heating integration
- Staircase alterations (if required)
Finishes such as kitchens, flooring, decorating, and landscaping are priced depending on specification.
How Much Value Does a Two-Storey Extension Add?
A well-designed two-storey extension can increase property value by 15–25%, depending on size and layout improvements.
For many homeowners, this makes extending more financially sensible than moving—especially when considering:
- Stamp duty
- Legal fees
- Estate agent fees
- Removal costs
Example Scenario (Hertfordshire / Essex)
A common project we complete is:
- Ground floor: Kitchen-diner extension
- First floor: Additional bedroom and en-suite
Approximate 60m² total build cost:
£120,000 – £150,000, depending on finish level.
Every home is different, so accurate pricing requires a site visit and consultation.
Is It Cheaper to Build Up or Out?
If space allows, building up (two storeys) often offers better long-term value than extending outward twice. However, planning constraints, garden size, and neighbouring properties all influence the best option.
This is why early consultation is essential.
How to Keep Costs Under Control
- Finalise layout before building starts
- Choose specification early
- Work with an experienced builder
- Avoid mid-build design changes
- Plan during winter to secure preferred scheduling
At Garnet Construction, we provide transparent quotations with detailed breakdowns so there are no surprises.
Why Choose Garnet Construction?
We specialise in:
- Two-storey extensions
- Kitchen extensions
- Loft conversions
- Garage conversions
- Full home renovations
Serving homeowners across Hertfordshire and Essex, we focus on:
- Clear communication
- High-quality workmanship
- Realistic timelines
- Energy-efficient construction
- Fully compliant builds
Thinking About a Two-Storey Extension?
If you’re researching costs, the next step is a professional consultation tailored to your home.
The sooner you start planning, the sooner you can enjoy a larger, warmer, more valuable home.