If you’re planning a house extension in Cardiff or South Wales, it’s normal to ask:
“Do I need planning permission?”
In some cases, you may be able to extend under permitted development, but many projects require planning permission depending on the property type, location and design.
This guide explains what to consider before starting work.
When Planning Permission May Be Required
You may need planning permission if:
The extension is large or changes the shape of the house significantly
You’re in a conservation area
The property is a flat or maisonette (often different rules apply)
You’re extending close to a boundary
There are restrictions on the property (previous permissions / conditions)
Permitted Development (Basic Overview)
Some homeowners can build certain extensions without planning permission, but there are limits on:
Height
Depth
Materials
Position relative to the house and boundaries
✅ Even if planning permission isn’t needed, Building Regulations approval usually still is.
Building Regulations (Often Required)
Most extensions require Building Control sign-off to ensure:
Structural safety
Fire safety
Insulation standards
Drainage and ventilation compliance
How Long Does Planning Permission Take?
Timeframes vary, but a rough guide is:
Preparing drawings: 1–3 weeks
Planning application decision: often 8+ weeks
Building Regulations: can run alongside the process
Common Mistakes to Avoid
1) Starting too early
Starting work before approvals can lead to delays, additional costs, or enforcement issues.
2) Not considering neighbours
It’s always best to consider impact on:
light
privacy
access
3) Not planning the internal layout properly
Extensions work best when the layout is planned with the existing home in mind.
FAQs: Planning Permission
Can you build an extension without planning permission?
Sometimes, yes — but it depends on the property and extension design.
Do you help with the planning process?
We can guide you through the process and work alongside your architect or designer.
Thinking about an extension in Cardiff? Contact Mark James Building Contractors Ltd for advice and a free quote.