House extension under construction with scaffolding — All Aspects Construction, Cardiff

Ask five builders how long an extension takes and you will get five answers, all of them roughly “eight to twelve weeks”, and all of them measuring the wrong thing.

Eight to twelve weeks is the build. What determines when you actually get your kitchen back is everything that happens before anyone turns up with a digger — and that part varies from about a fortnight to about five months depending on decisions you make in the first conversation.

On this page Getting to a start date · A realistic programme · Where programmes slip · What we commit to

The part nobody counts: getting to a start date

There are three approvals that can sit in front of a build in this part of Wales. Only one of them is on everyone’s radar.

Planning permission — or not needing it

This is the big one. If the design fits within permitted development, there is no planning application and no planning wait. If it does not, you are into a full application, and that is the difference between starting next month and starting after the summer.

Single-storey rear extension limits in Wales In Wales a single-storey rear extension may project up to 4 metres beyond the original rear wall and be up to 4 metres high, with no separate allowance for detached houses. England's Larger Home Extension prior-approval scheme, which permits 6 metres on an attached house and 8 metres on a detached one, does not apply in Wales. How far a single-storey rear extension can go — in Wales existing house original rear wall 4m +2m 4m max height England only The 6m / 8m “Larger Home Extension” prior-approval route does not exist in Wales. permitted development in Wales needs full planning permission here
Wales applies one 4m limit to every house type. There is no detached-house allowance and no prior-approval route to 6m or 8m — those are England’s rules. Source: gov.wales, “Planning permission: extensions”.

In Wales the permitted development limit for a single-storey rear extension is 4m beyond the original rear wall and 4m in height. There is no prior approval route to 6m or 8m — that is England only. Going to a conservation area address removes more rights again: no roof extension, no side extension, no rear extension above single storey.

The practical consequence is that a 4.5m extension and a 4m extension can have completely different start dates for the sake of half a metre. That is worth knowing at sketch stage, which is why we check it before anyone commissions drawings.

Building control

Separate from planning, and applies either way. Permitted development is not exempt from building regulations — it only means you did not need planning permission. This usually runs alongside the build via staged inspections rather than blocking the start.

The one that catches Caerphilly and Pontypridd out

If you are on the old coalfield and the ground works would disturb coal seams, abandoned workings or a mine entry — piling, deep trenching, drilling, grouting — that needs a permit from the Mining Remediation Authority, and a standard application takes 20 or more working days.

What a homeowner on the old coalfield actually needs Householder development such as a house extension is exempt from the Coal Mining Risk Assessment requirement, even inside a Development High Risk Area. A separate permit from the Mining Remediation Authority, formerly the Coal Authority, is required only where ground works would disturb coal seams, abandoned workings or mine entries, and takes 20 or more working days. Extending on the old coalfield: what you need, and what you don’t Your home is in a Development High Risk Area Is it householder development — an extension, a loft? yes No Coal Mining Risk Assessment needed householder applications are exempt by type A SEPARATE QUESTION Will the ground works disturb workings, seams or a mine entry? yes — piling, drilling, grouting Mining Remediation Authority permit required standard applications take 20+ working days — build it into the programme, not the panic The Coal Authority was renamed the Mining Remediation Authority. Most trade sites still use the old name. Sources: GOV.UK planning applications and Coal Mining Risk Assessments; GOV.UK permits to deal with coal or coal mines.
Two different questions get muddled constantly. Planning does not ask a homeowner for a mining risk assessment. The permit is about physically disturbing old workings, and it is the one with a lead time worth planning around.

Note what the diagram also says: a householder extension is exempt from the Coal Mining Risk Assessment that most planning applications in a Development High Risk Area require. Plenty of homeowners are told they need one and pay for it. They do not. The permit is the real item, it only applies if the ground works actually go that way, and it belongs in the programme from week one rather than being discovered mid-dig.

A realistic programme for a single-storey rear extension

Assuming a straightforward permitted development job on a reasonably accessible site:

Stage Typical duration
First visit, measure, and confirm what the rules allow at your address1–2 weeks
Drawings and structural calculations2–4 weeks
Building control application and notice periodsruns in parallel
Party wall notices, where a neighbour’s wall or boundary is involvedallow 2 months from service
Groundworks and foundations2–3 weeks
Structure, walls and roof to watertight3–4 weeks
First fix, plastering, second fix3–4 weeks
Finishes, kitchen, decoration and snagging2–3 weeks
The same programme drawn on a calendar The stages of a straightforward permitted development extension shown against a twenty week timeline. Roughly three to five weeks of site visit, drawings and structural calculations come before any work starts. Building control runs alongside rather than blocking the start. Party wall notices, where they are needed, take about two months from service and are the item most likely to hold up a start date. The build itself, from groundworks through to snagging, runs about eight to twelve weeks. A full planning application sits in front of all of it and adds months rather than weeks. A straightforward permitted development extension, on a calendar wk 0 4 8 12 16 20 Site visit & rules check Drawings & calcs Building control runs alongside — does not block the start Party wall notices allow 2 months from service — serve them early SITE STARTS Groundworks Structure to watertight First & second fix Finishes & snagging shortest typical the longer end of the same stage Add a full planning application in front of all of this and you are adding months, not weeks. Durations as set out in the table above. Every job is different — this is the shape, not a promise.
The same durations as the table, drawn against a calendar. It shows the two things a list cannot: what runs alongside the build rather than in front of it, and how far the party wall clock reaches if the notices are served late.

Add a full planning application and you are adding months, not weeks — the determination period is only part of it, because validation, consultation and any requested amendments all sit on top. This is the single biggest reason to establish whether your scheme fits permitted development before you fall in love with a drawing.

Where programmes actually slip

Party wall notices served late

On the terraced streets of Barry, Cadoxton, Canton and the Pontypridd valley sides, party wall matters are the norm rather than the exception. The notice periods are statutory and cannot be compressed, so serving them late does not delay a week — it delays the whole start.

Loft conversion roof trusses timber structure — All Aspects Construction, Cardiff
Roof structure going on at one of our jobs. Getting to watertight is the stage weather can move, which is why we would rather quote a range than a date we cannot control.

Decisions that arrive after the wall is up

Changing the position of a socket during first fix costs minutes. Changing the position of a window after the lintel is in costs days. We front-load the decisions that are expensive to reverse and leave the genuinely late ones — paint, handles, tiling layout — late.

Freshly plastered and finished room during renovation — All Aspects Construction, Cardiff
The last two or three weeks on our own jobs: plaster dry, second fix done, decoration and snagging. Nothing structural happens here — but this is the part the client actually sees.

Ground that is not what the ground was supposed to be

Old drainage, made ground, a level change nobody measured, or workings on a valley-side plot. This is the one honest source of variation that no programme can fully remove, and it is why we would rather dig a trial hole early than write an optimistic date.

What we commit to

A programme with the approvals on it, not just the build. If a permit or an application is going to sit in front of your start date, you will know at the first visit — not in week three when the digger is booked and idle.

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