If you’re considering a house extension in Cardiff or the surrounding areas, one of the first questions you’ll ask is: “How much is it going to cost?”
The honest answer is that extension costs vary depending on the size, design, finish, access and ground conditions — but this guide will give you a clear idea of typical prices and what impacts your budget most.
At Mark James Building Contractors Ltd, we build extensions across Cardiff and South Wales (within roughly a 1 hour radius) and provide clear, detailed quotations so homeowners understand what they’re paying for.
Typical House Extension Costs in Cardiff (Guide Prices)
Here are rough guide prices for common extension types:
Single Storey Extension
Most homeowners choose a single storey extension for:
Kitchen/diner expansions
Open-plan living space
Utility rooms and downstairs WCs
Guide cost range: £35,000 – £85,000+
(depending on size and specification)
Double Storey Extension
A double storey extension is ideal for:
Additional bedrooms
Enlarged bathrooms
Better family living space
Guide cost range: £70,000 – £140,000+
Wraparound / Side Return Extensions
These often create the biggest impact internally, especially for older Cardiff properties.
Guide cost range: £60,000 – £120,000+
What Affects the Cost of an Extension Most?
1) Size and layout
A larger extension needs more foundations, structure, roofing, labour and finishing.
2) Groundworks and drainage
Ground conditions, drainage runs, and access for machinery can significantly affect costs.
3) Structural steelwork
If you’re opening up the rear of the house into the extension, you may need:
Steel beams
RSJs
Structural calculations
4) Roof type
Flat roofs can be cheaper short-term, but pitched roofs may suit the property better long term depending on design and materials.
5) Windows and doors
Large bifold doors, skylights, aluminium frames or premium glazing will increase costs.
6) Internal finishes
For example:
Basic plaster + paint vs full kitchen fit-out
Standard flooring vs luxury finishes
7) Access and site restrictions
If access is tight, labour and waste removal can take longer and increase costs.
What’s Usually Included in an Extension Quote?
A professional quote should clearly outline items such as:
Foundations and groundworks
Brickwork / blockwork / structure
Roofing and insulation
Windows and external doors
Plastering and internal joinery
Electrics and plumbing (where required)
Waste removal and skips
Site management and labour
If something is excluded (like kitchen supply), it should be clearly stated.
How to Budget Properly
We recommend allowing for:
✅ A contingency (typically 5–10%)
✅ Design and approvals (architect / drawings / Building Control)
✅ Finishing items (flooring, kitchen choices, lighting upgrades)
FAQs: House Extension Costs
How can I reduce extension costs without cutting corners?
Good planning is the biggest cost saver. Finalise layouts, finishes and structural design early to minimise changes during the build.
Is a single storey extension worth it?
For many Cardiff homes, yes — it can transform layout and lifestyle and often increases the home’s value.
Do you offer free quotes?
Yes — we provide a site visit and quotation for extensions in Cardiff and surrounding areas.
If you’re planning an extension in Cardiff or South Wales, contact Mark James Building Contractors Ltd today to arrange a free quote and site visit.