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 Colwyn Bay?
In Colwyn Bay, a double-storey extension typically costs £1,500–£2,300 per m² of floor area (both floors), including VAT at 20% (standard rate). For a typical 36 m² project that is roughly £54,000–£83,000. Building costs in Wales run about 8% below the UK average, and the figures for Colwyn Bay are adjusted to reflect that. Design, structural calculations and council fees are extra.
| Size or type | Guide range |
|---|---|
| Smaller project (25 m²) | £37,500–£57,500 |
| Typical project (36 m²) | £54,000–£83,000 |
| Larger project (50 m²) | £75,000–£115,000 |
| Rate per m² | £1,500–£2,300 |
The main things that change the cost:
- Total floor area across both floors
- Roof design and how it joins the existing roof
- Scaffolding and access to the rear
- Steel beams where walls are opened up
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 Colwyn Bay?
Before anything is built in Colwyn Bay, we confirm which approvals apply in Wales. In Wales a rear extension of more than one storey can be permitted development if the ground floor projects no more than 4 metres and the upper floor no more than 3 metres, with at least 10.5 metres to the rear boundary and obscure-glazed upper side windows. It is not permitted development in conservation areas, National Parks or National Landscapes, so many double-storey extensions need a householder application.
Building regulations in Wales are set by the Welsh Government. Approval comes from the council’s building control service or a registered building control approver, with inspections during the work and a completion certificate 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
Handled as part of most double-storey extensions:
- Structural engineer’s calculations
- Opening up at both levels with steel beams
- Windows, doors and staircase alterations
- Building control sign-off
- Brick or block walls, floors and a roof tied into the house
- Design drawings and planning application
How long does it take?
Allow 14 to 20 weeks on site, after planning and building control approval. Allow time beforehand for drawings and approvals, usually 3 to 4 months.
Building in Colwyn Bay
Terraced houses in Welsh towns often have narrow plots, which suits rear and side return extensions and rear dormers.
Projects in Colwyn Bay (LL29) are surveyed in person before we quote, so the price reflects your actual property.
How it works
- 01
Free visit
We look at the property, talk through what you want and measure up. There is no charge and no obligation.
- 02
Design and fixed quote
Drawings, structural calculations and an itemised, fixed price for the agreed 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 Colwyn Bay
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.
Is a double-storey extension cheaper per square metre?
Usually yes. You pay for one set of foundations and one roof but get two floors of space, so the cost per square metre is often lower than a single-storey extension.
Who approves a double-storey extension in Colwyn Bay?
Planning matters in Colwyn Bay go to the council covering your address, and the work needs building control approval. We prepare and submit the applications for you.
How much does a double-storey extension cost in Colwyn Bay?
In Colwyn Bay, a double-storey extension typically costs £1,500–£2,300 per m² of floor area (both floors), including VAT at 20% (standard rate). A typical 36 m² project comes to roughly £54,000–£83,000. 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.
Do I need planning permission for a double-storey extension in Colwyn Bay?
In Wales a rear extension of more than one storey can be permitted development if the ground floor projects no more than 4 metres and the upper floor no more than 3 metres, with at least 10.5 metres to the rear boundary and obscure-glazed upper side windows. It is not permitted development in conservation areas, National Parks or National Landscapes, so many double-storey extensions need a householder application.