Extension cost per m² in Cardiff
Extension cost by size in Cardiff
Each row multiplies the current £1,800–£3,000 per m² national benchmark by floor area and then applies the same regional factor.
What can change a real Cardiff quote?
- Floor area and number of storeys
- London and regional labour rates
- Steelwork and structural changes
- Kitchen or bathroom installation
- Glazing and door specification
- Finish level, access and contingency
Local planning considerations
For grants and energy schemes, verify the current Wales-specific eligibility rules before relying on any support assumption.
- Use Wales-specific planning, building-control and energy-scheme information where the rules differ from England.
- Terraced or constrained urban properties can add access, waste and scaffold costs that are not captured by the regional factor.
- Drainage, roof and external-work scopes should be based on the property condition rather than a generic city assumption.
How the regional model works
The 0.91× factor places this planning estimate 9% below the national planning baseline. We keep that modelling assumption separate from the dated national benchmark so users can see which part comes from an external source and which part is UK Home Costs modelling.
Sources
House Extension: Cost Breakdown 2026
Published / checked: June 2026
Used for: Extension cost-per-m² benchmarks and common add-on costs
Construction output price indices
Published / checked: 13 August 2026
Used for: Construction price trend and update checks
Frequently asked questions
How much does a house extension cost in Cardiff in 2026?
Our modelled Cardiff planning rate is about £1,650–£2,750 / m², based on the current national £1,800–£3,000 per m² benchmark and the Wales regional factor. This is not an observed city-wide quote average.
How much is a 20m² extension in Cardiff?
Applying the same regional model gives about £32,750–£54,600 before VAT for a straight-line 20m² example. Structural work, glazing, kitchens, bathrooms and access can move a real quote outside that range.
Why is the Cardiff estimate different from the UK benchmark?
UK Home Costs applies a 0.91× Wales planning factor, placing the model 9% below the national planning baseline. It is our modelling assumption rather than an external city price index.
Does the extension figure include VAT and professional fees?
The national benchmark is stated before VAT. Do not assume architect, structural engineer, planning, building-control, party-wall or specialist survey fees are included unless a contractor quote says so.