Types of house extension
Single-storey rear extension — The most common extension type. Extends the kitchen or living space to the rear of the property. Subject to Permitted Development rules in most cases.
Two-storey rear extension — More expensive, adds an upper bedroom or bathroom above the ground floor extension. Almost always requires planning permission. Requires structural engineering.
Side return extension — Popular in Victorian terraced properties with a narrow side passage. Typically widens the ground floor kitchen/dining area. Permitted Development in most cases.
Wrap-around extension — A combination of side return and rear extension, creating an L-shaped addition. Significant space gain but higher cost.
Single-storey front extension — Less common, adds a porch or ground floor room to the front elevation. Usually requires planning permission.
Extension costs in Wales 2026
Extension costs in Wales vary significantly by location and specification. Cardiff and Newport are at the higher end; Valleys and rural areas are typically 15–25% lower.
Single-storey rear extension (per m²):
- Basic finish (plastered, ready for decoration): £1,200–1,600/m²
- Mid-range finish (fitted, decorated, finished): £1,600–2,100/m²
- High-end finish (premium spec): £2,100–2,800/m²
Typical total costs for common extension sizes:
| Extension size | Basic | Mid-range | High-end |
|---|---|---|---|
| 10m² (small kitchen) | £12,000–16,000 | £16,000–21,000 | £21,000–28,000 |
| 20m² (medium kitchen-diner) | £24,000–32,000 | £32,000–42,000 | £42,000–56,000 |
| 30m² (large open-plan) | £36,000–48,000 | £48,000–63,000 | £63,000–84,000 |
Two-storey extension: Add 50–80% to the single-storey cost for equivalent floor area.
These costs include structural work, damp-proof course, insulation, windows and doors, plastering, and basic electrical and plumbing first fix. They typically exclude architect fees, structural engineering, building regulations fees, kitchen or bathroom fitting, and external landscaping.
Professional fees to budget separately:
- Architect drawings: £1,500–4,000
- Structural engineer: £800–2,000
- Building regulations application (Wales): £500–1,200
- Party wall surveyor (if applicable): £700–1,500 per neighbour
Extension costs by Welsh city
Extension prices vary across Wales by 15–30% depending on local builder rates, transport costs to site, and competition. Below are typical 20m² single-storey rear extension costs (mid-range finish) in the five biggest Welsh cities as of 2026:
| City | Typical 20m² extension (mid-range) | Why it varies |
|---|---|---|
| Cardiff | £36,000–44,000 | Highest demand, higher builder day rates, parking restrictions add cost |
| Swansea | £32,000–40,000 | Active building scene, competitive quotes from established firms |
| Newport | £30,000–38,000 | Lower overheads than Cardiff, easy site access in most postcodes |
| Wrexham | £28,000–36,000 | Lower labour rates than south Wales, materials similar |
| Bangor | £30,000–38,000 | Materials transport from Manchester/Liverpool adds 5–10% |
Tip: if you live within 15 miles of Cardiff but in a Vale of Glamorgan or Caerphilly postcode, expect to pay closer to the Cardiff rate — builders price by where their team is based, not where you live. Always confirm whether quotes include VAT — standalone new-build extensions are zero-rated in some scenarios but reconfigurations of existing rooms are not. Ask the builder to break it out.
For typical 30m² wrap-around extensions, add 35–50% to the figures above. For two-storey extensions of the same floor footprint, add 50–80%.
Planning permission in Wales
Permitted Development allows many single-storey rear extensions without a planning application, subject to size and location limits. Key rules:
- Rear extension no more than 4m deep (detached) or 3m (attached)
- No higher than 4m or the existing eaves height
- Does not cover more than half the garden area
- Not in a conservation area or SSSI
Wales has slightly different PD rules from England in some cases — always check with your local planning authority (Cardiff Council, Newport Council, etc.) or use a permitted development certificate application to get certainty.
Extensions always requiring planning permission:
- Two-storey extensions in most cases
- Front extensions affecting the street scene
- Properties in conservation areas
- Listed buildings (also need listed building consent)
Hiring a builder for an extension
An extension is a significant investment. Key steps before work starts:
Get architect drawings — Even for Permitted Development extensions, good working drawings are essential for pricing. They also help you visualise the result and spot issues before construction.
Get structural engineering calculations — Any extension with a loadbearing wall, RSJ beam, or significant structural element needs a structural engineer's calculations. Budget this separately from the builder.
Get three quotes — Extension quotes can vary 25–40% between builders for the same job. Get three quotes from builders who have done similar projects locally.
Check insurance and references — Builder's public liability and employers' liability insurance are essential. Ask for references from completed extension projects — ring the references and, ideally, visit the completed work.
Stage payments — Pay in agreed stages tied to completion milestones, not a percentage upfront. A common structure: 10% deposit, 25% at foundation completion, 25% at wall plate height, 25% at plastering completion, 15% on final handover.