Home extension builders in Cardiff
House prices across Cardiff have made extending the property you already own far more cost-effective than buying somewhere bigger. A well-built rear extension on a terrace in Roath, Canton or Pontprennau usually adds more usable space — and more resale value — than the difference in price to the next rung of the property ladder.
J&Z Property Maintenance builds home extensions and conversions across Cardiff. We handle the full project: planning permission discussions, structural design coordination, groundwork, build, insulation, plastering and first fix — under one contract, with one team and one point of contact throughout. No managing multiple contractors. No invoices you were not expecting.
Every project starts with a free site visit and a full written quote. We do not start work until you have approved the cost, the scope and the timeline.
Extension and conversion types we build
Rear extensions
The most popular way to extend a Cardiff home — opening up the back of a terraced or semi-detached property to create an open-plan kitchen and dining space. Single or double storey, managed from groundwork to plastered finish.
Wraparound extensions
A side return and rear extension combined, particularly effective on terraces with an underused side alley. Dramatically increases ground floor space and natural light. We assess permitted development eligibility before any design work starts.
Side return extensions
The narrow passage between a terraced house and its boundary wall is wasted space. A side return reclaims it — typically widening a galley kitchen or creating a downstairs bathroom. Common across Canton, Pontcanna and Roath.
Double-storey extensions
Adds space on both floors at once — a larger kitchen below and an extra bedroom above. More complex planning considerations apply and it almost always needs full planning permission in Wales. We advise from the outset and manage the application.
Loft conversions
Convert unused loft space into a bedroom, bathroom or home office without losing any garden. Dormer conversions suit Cardiff's Victorian and Edwardian terraces best. We handle structural calculations, joists, stairs, insulation and fit-out.
Garage conversions
Turn an integral or attached garage into a home office, bedroom, utility or playroom. Often the cheapest way to add a room because the structure already exists.
Garage conversions Cardiff →
Important: the extension advice you read online is probably English
Planning rules in Wales are not the same as England, and most extension guidance online is written for an English audience. Following it will give you the wrong answer for a Cardiff property.
The key difference is this. In England there is a prior approval route — the larger home extension scheme — that lets homeowners build single-storey rear extensions well beyond the standard permitted development limits without a full planning application. Wales never adopted that scheme.
If your extension exceeds the Welsh permitted development limits, you need full planning permission from Cardiff Council. There is no shortcut.
Welsh permitted development also applies stricter conditions on things like the proportion of garden you can build over, eaves height near boundaries, and materials on the principal elevation. We assess all of this at the first site visit, at no charge, so you know from the outset whether you are looking at a permitted development project or a planning application. This is general guidance rather than formal planning advice — for anything unusual or borderline we will tell you to speak to Cardiff Council planning or a planning consultant.
Planning permission for Cardiff extensions
Often permitted development
- Single-storey rear extensions within Welsh size limits
- Most loft conversions using existing roof space
- Velux and rooflight loft conversions
- Garage conversions retaining the existing structure
- Extensions not fronting a highway
- Work not exceeding the height of the existing roof
Usually needs planning permission
- Double-storey extensions
- Anything exceeding Welsh permitted development limits
- Properties in conservation areas
- Listed buildings and their curtilage
- Larger dormer conversions altering the roofline
- Extensions altering the principal elevation
Permitted development rights can also be removed by an Article 4 direction or a planning condition on newer estates — worth checking before you plan anything. We check this as part of the free site visit.
How much does an extension cost in Cardiff?
| Extension or conversion |
Typical Cardiff range |
Key variables |
| Single-storey rear extension |
£20,000 – £45,000 |
Size, glazing spec, kitchen fit-out if included |
| Side return extension |
£20,000 – £40,000 |
Length of return, glazed roof option |
| Wraparound extension |
£35,000 – £65,000 |
Complexity of combining side return and rear |
| Double-storey extension |
£40,000 – £80,000 |
Structural complexity, bedroom count, bathroom spec |
| Loft conversion (Velux) |
£18,000 – £28,000 |
Number of rooflights, staircase design, head height |
| Loft conversion (dormer) |
£28,000 – £45,000 |
Dormer size, en-suite inclusion, structural work |
| Garage conversion |
£8,000 – £18,000 |
See garage conversions Cardiff
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All figures are indicative ranges based on typical Cardiff residential projects. Every project is quoted individually following a free site visit. Your written quote includes all labour, materials and project management — no hidden costs.
Why choose J&Z for your Cardiff extension
One team, one contract
Most extension builders subcontract groundwork, brickwork, roofing and electrics to separate trades. We manage all of it — one contract for the full project, one team on site throughout, one number to call if anything changes.
Welsh planning advice before the quote
We assess permitted development eligibility under Welsh rules on the first visit. If you need a planning application we tell you upfront and can manage it. No surprises after you have committed.
Built for Welsh weather
Cardiff's rainfall and exposure mean insulation specification, damp-proofing and drainage detail matter more than in drier parts of the UK. We specify and build to what the site needs, not to the minimum standard.
Written quote, fixed scope
Every project is quoted in full before work starts and the scope is agreed in writing. If something changes on site we discuss it with you before it affects your cost.
Frequently asked questions — extensions & conversions Cardiff
Do I need planning permission for a rear extension in Cardiff?
It depends on size. Single-storey rear extensions within the Welsh permitted development limits generally do not need a planning application. The important thing to know is that Wales did not adopt England's larger home extension prior approval scheme, so if your extension goes beyond the permitted development limits you need full planning permission from Cardiff Council — there is no prior approval shortcut as there is across the border. Conservation areas and listed buildings have additional restrictions. We assess your eligibility during the free site visit.
Why is planning advice for England wrong for Cardiff?
Because permitted development rights are devolved and the two nations diverged. The most significant difference for homeowners is the larger home extension scheme, which exists in England and does not exist in Wales. Welsh rules also differ on garden coverage, eaves heights near boundaries and materials on the principal elevation. If you have read that you can build a 6 metre rear extension under prior approval, that is English guidance and it does not apply to your Cardiff property.
How long does a home extension in Cardiff take?
A single-storey rear extension typically takes 10–16 weeks from groundwork to completion. A loft conversion is usually 8–12 weeks. A double-storey extension runs 16–24 weeks depending on structural complexity. These timelines assume planning permission is already in place where required — allow a further 8–12 weeks for a planning application through Cardiff Council if one is needed.
Can you manage the full project including planning?
Yes. We handle the project from initial design coordination through to final finish. Where a planning application is required we manage that process on your behalf including drawings, submission and correspondence with Cardiff Council. You do not need to coordinate separate architects, structural engineers and builders — we bring it together.
What is the difference between a dormer and a Velux loft conversion?
A Velux conversion installs rooflights into the existing roofline without altering the roof structure. It is less disruptive and less expensive but only works where there is already sufficient head height. A dormer builds out from the roof to create a vertical-walled space, adding significantly more usable floor area and head height. Most Cardiff terraces benefit more from a dormer, but we assess both options on site and advise accordingly.
Will I need to tell my neighbours about the work?
Often yes. Extension work involving a shared wall — cutting in to seat a beam, or excavating foundations near a neighbouring property — is notifiable under the Party Wall etc. Act 1996, generally requiring two months' notice. This is separate from planning permission and applies even where planning consent is not needed. We identify whether your project is notifiable at the quoting stage. For disputed cases you should speak to a party wall surveyor.
Do you work across all Cardiff postcodes?
Yes — all Cardiff postcodes CF1 through CF24 and surrounding areas including Barry, Penarth, Caerphilly and Pontypridd. Most of our extension work is concentrated in inner Cardiff — Canton, Pontcanna, Roath, Cathays and Llandaff — where the Victorian and Edwardian terraced stock suits rear and wraparound extensions particularly well.
Other services we offer in Cardiff
Extensions and conversions across Cardiff and South Wales
Cardiff · Canton · Pontcanna · Roath · Cathays · Llandaff · Whitchurch · Grangetown · Pontprennau · Barry
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Penarth · Caerphilly · Pontypridd · Newport
Get a free extension estimate in Cardiff
Call us or send your details and we will arrange a free site visit — evenings and weekends available, no charge, no obligation.
