What is a double-storey extension?
A double-storey extension (also called a two-storey extension) adds rooms on both the ground and first floor in one build. It typically gives a larger kitchen or living space downstairs and an extra bedroom or bathroom above.
How much does a double-storey extension cost in Sunbury-on-Thames?
In Sunbury-on-Thames, a double-storey extension typically costs £1,850–£2,800 per m² of floor area (both floors), including VAT at 20% (standard rate). For a typical 36 m² project that is roughly £66,500–£101,000. We start from a UK-wide guide and adjust it for the South East, where labour and materials are typically about 12% above average. Design, structural calculations and council fees are extra.
| Size or type | Guide range |
|---|---|
| Smaller project (25 m²) | £46,000–£70,000 |
| Typical project (36 m²) | £66,500–£101,000 |
| Larger project (50 m²) | £92,500–£140,000 |
| Rate per m² | £1,850–£2,800 |
The main things that change the cost:
- Brick matching and external finishes
- Scaffolding and access to the rear
- Bathrooms or en-suites on the first floor
- Total floor area across both floors
Use the guide price tool to see how size and specification change the figure, then ask for a written, itemised quote.
Do you need planning permission for a double-storey extension in Sunbury-on-Thames?
Before anything is built in Sunbury-on-Thames, we confirm which approvals apply in England. In England an extension of more than one storey can only be permitted development at the rear, no more than 3 metres deep and at least 7 metres from the rear boundary, with matching materials and any upper side windows obscure-glazed. Most double-storey extensions therefore go through a householder planning application.
Building regulations approval comes from the council’s building control service or a registered building control approver. The work is inspected at key stages, such as foundations, drainage and insulation, and a completion certificate is issued at the end.
At the survey we check your house’s planning history and any restrictions, and tell you which route applies.
What we handle
From survey to sign-off, the job normally involves:
- Foundations, drainage and scaffolding
- Opening up at both levels with steel beams
- Building control sign-off
- Windows, doors and staircase alterations
- Brick or block walls, floors and a roof tied into the house
- Structural engineer’s calculations
How long does it take?
The build itself usually takes 14 to 20 weeks on site, after planning and building control approval. Design and approvals come first and typically take 3 to 4 months, depending on the approval route.
Building in Sunbury-on-Thames
Commuter towns across the South East have many 1930s and post-war semis that suit rear extensions and hip-to-gable loft conversions.
We work across Sunbury-on-Thames and neighbouring areas such as Walton-on-Thames, Ashford and Weybridge. Postcodes we cover in Sunbury-on-Thames include KT12 and TW16.
How it works
- 01
Visit and measured survey
A free visit to understand the project, check access and take measurements.
- 02
Design and pricing
Design options, then a fixed quote based on the chosen drawings and specification.
- 03
Approvals in place
We handle planning, building control and neighbour notices before work starts.
- 04
Build and handover
Work follows the agreed programme, with inspections along the way and a completion certificate at the end.
Questions about double-storey extensions in Sunbury-on-Thames
How much does a double-storey extension cost in Sunbury-on-Thames?
In Sunbury-on-Thames, a double-storey extension typically costs £1,850–£2,800 per m² of floor area (both floors), including VAT at 20% (standard rate). A typical 36 m² project comes to roughly £66,500–£101,000. Your fixed price depends on the survey and drawings.
Will the first floor need a new staircase?
Not normally. The new rooms are reached from the existing landing, though a doorway or short corridor may need to be formed.
How long does a double-storey extension take in Sunbury-on-Thames?
Typically 14 to 20 weeks on site, after planning and building control approval. Design and approvals beforehand usually take 3 to 4 months.
Can I live at home during the work?
Most families stay put. The shell is built first and the house is opened up late in the job, so disruption inside is limited to a few weeks.
What drawings do I need?
Planning drawings for the application, then more detailed building regulations drawings and structural calculations for building control. We prepare both.