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House Extensions in Wiltshire
House extensions are the largest thing most people ever do to a home, and the part that goes wrong is rarely the building work — it is the order things happen in. This covers the types, what they cost in Wiltshire, when you need planning permission, and where the carpentry fits.
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Where we fit
What a carpenter does on an extension
An extension is not one trade. It is groundworks, brickwork, roofing, carpentry, plumbing, electrics and plastering, in that rough order, and the thing that goes wrong is almost never the individual work — it is the sequencing.
We are the carpenter. On the first fix that means the structural timber: floor joists, wall plates, roof carpentry, trusses or a cut roof, studwork partitions, and the openings formed for windows and doors. On the second fix it means doors hung, skirting and architrave, a staircase if the extension has one, and the fitted joinery that makes the new space usable.
You will want a main builder for the groundworks, brickwork and roof covering, and we work alongside them regularly. If you have not got one yet, we can tell you what order things need to happen in and what to ask for.
What follows is the rest of it: the types, what they cost in Wiltshire, and the planning and Building Regulations rules that decide what you are allowed to build.
Types
Types of house extension
Single-storey rear extension
The most common by a distance, and usually the one that creates a kitchen-diner across the back of the house. Often permitted development. Cheapest per square metre when it is a simple rectangle with a flat or mono-pitch roof; the cost climbs quickly with glazing.
Side extension
Filling the gap down the side of a semi-detached or detached house. Permitted development limits it to half the width of the original house and single storey, with a maximum height of 4 metres. Good value where the side return is otherwise dead ground.
Wrap-around
Rear and side combined, usually round an L-shaped Victorian plan. Almost always needs full planning permission because it exceeds permitted development on at least one axis, but it transforms the ground floor more than anything else.
Two-storey extension
Costs less per square metre than single storey, because the expensive parts — foundations and roof — are shared between two floors. Permitted development allows up to 3 metres to the rear on a detached or semi, provided it is more than 7 metres from the rear boundary.
Garage conversion
The cheapest way to add a room, because the walls, roof and slab already exist. The work is infilling the door opening, a damp-proof and insulated floor build-up, insulating the walls and roof, and adding heating and glazing. Usually permitted development if it stays within the existing footprint.
Garden rooms and orangeries
A separate detached building in the garden avoids most extension rules entirely — see garden offices. An orangery is an extension with a lantern roof, and is treated as an extension for both planning and Building Regulations.
Cost
What a house extension costs in Wiltshire
Whole-project figures across all trades, for a decent standard of finish. Our carpentry is one line within these, not the total.
| Type | Typical cost |
|---|---|
| Garage conversion | £1,000 – £1,500 / m² |
| Single-storey rear extension | £2,000 – £3,000 / m² |
| Single-storey with large glazing or bi-folds | £2,600 – £3,600 / m² |
| Two-storey extension | £1,800 – £2,600 / m² |
| Wrap-around | £2,400 – £3,400 / m² |
A 20m² kitchen extension at the middle of that range lands around £40,000–£60,000. On top: architect or designer at roughly 5–10 per cent, structural engineer £500–£1,500, Building Control £600–£1,200, planning application £258 for a householder application, and party wall surveyors if it comes to that. What moves the number most is glazing, groundworks on a sloping or clay site, and moving the drains.
Planning
Planning permission and permitted development
Do you need planning permission?
Often not. Permitted development rights let you build a single-storey rear extension of up to 4 metres on a detached house or 3 metres on a semi-detached or terraced one, without any application at all — provided the height stays under 4 metres, the eaves match the existing where within 2 metres of a boundary, and the extension plus any other outbuildings covers no more than half the original garden.
Prior Approval — the longer route
Larger single-storey rear extensions — up to 8 metres on a detached house and 6 metres on a semi or terrace — are allowed under the Prior Approval procedure. You notify the council, they consult your neighbours, and they have 42 days to respond. If a neighbour objects, the council decides whether the impact is acceptable. It is faster and cheaper than full planning permission, and it is widely under-used because most people have never heard of it.
When you always need full permission
Conservation areas and listed buildings, which cover a great deal of central Bradford on Avon, Devizes, Salisbury and Trowbridge. Flats and maisonettes, which have no permitted development rights. Anything forward of the principal elevation. Any extension where an Article 4 direction has removed permitted development rights. And anything exceeding the limits above.
The Party Wall Act
Separate from planning, frequently forgotten, and expensive to get wrong. You must serve notice under the Party Wall etc. Act 1996 if you are building on or at the boundary line, cutting into a shared wall, or excavating within 3 metres of a neighbour's structure to a depth below their foundations.
Notice periods are one month for boundary work and two months for work to a party wall. If your neighbour dissents, surveyors are appointed and an award is drawn up — typically £1,000–£2,000 per surveyor, paid by you. Serving notice late is one of the most common and most avoidable delays on any extension.
Get it in writing either way
Even where no permission is needed, apply for a Lawful Development Certificate. It costs about half a planning fee and it is the council confirming the work was permitted development. It is the document a buyer's solicitor will ask for, and without it an extension can stall a sale a decade later.
Building Regulations
What Building Control checks
Planning is about what it looks like and how it affects the neighbours. Building Regulations are about whether it stands up, keeps the heat in and does not burn. They apply to every extension, permitted development or not.
Foundations and structure
Depth is set by the ground and by nearby trees — clay soil with a mature oak forty feet away can mean 2.5 metres or a piled solution. Steel beams over new openings are sized by a structural engineer, and the calculations go to Building Control.
Part L — thermal performance
New walls around 0.18 W/m²K, floors 0.18, roofs 0.15, and windows and doors to 1.4 or better. In practice that means full-fill cavity insulation plus insulated plasterboard, and a floor build-up considerably thicker than people expect — which matters, because it sets the finished floor level and therefore the door threshold.
Part B — fire
Mains-wired interlinked smoke alarms, and escape windows to any habitable room that is not on the ground floor with its own exit. If the extension takes the house to three storeys the requirements step up substantially.
Drainage
Building over or near a public sewer needs a build-over agreement from Wessex Water, and it needs applying for early. Discovering a shared drain under your proposed footprint after the foundations are marked out is a bad day.
Ventilation and glazing
Background and extract ventilation to kitchens and bathrooms. Glazing in critical locations — doors, low panes, beside doors — must be toughened or laminated and marked as such.
Sign-off
Inspections at foundations, damp-proof course, structure, insulation and completion, then a completion certificate. Keep it with the Lawful Development Certificate — between them they are what proves the extension is legitimate.
The process
The order things need to happen in
Most extension disasters are sequencing, not workmanship. This is the order that works.
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Feasibility and design
What you want, what the site allows, and roughly what it costs. An architect or architectural technologist draws it; a structural engineer sizes the steels. Budget 4–10 weeks.
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Permissions
Planning or Prior Approval or a Lawful Development Certificate, Building Regulations application, party wall notices served, and a build-over agreement if there is a sewer. 8–16 weeks, and the notices are the part people leave too late.
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Build
Groundworks, structure, roof, then the trades in sequence. A single-storey rear extension is typically 10–16 weeks on site; two storeys 16–24. Our first fix carpentry starts once the shell is weathertight.
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Second fix and sign-off
Doors, skirting, architrave, fitted joinery, then plastering, decorating and the Building Control completion certificate.
Recent work
Structural and finishing carpentry
Photographs of this job to follow
Timber cladding to a rear elevation
A tired rendered rear wall re-clad in horizontal timber, battened out and ventilated behind, cut around an existing window and returned neatly into the door reveal.
Questions
House extensions — common questions
Do I need planning permission for an extension?
Often not. Permitted development allows a single-storey rear extension of up to 4 metres on a detached house or 3 metres on a semi or terrace, and up to 8 and 6 metres respectively under the Prior Approval route, where the council has 28 days to consider neighbours' objections. Height limits, eaves rules and the 50 per cent garden rule all apply, and conservation areas and listed buildings always need full permission.
What does a house extension cost in Wiltshire?
As of August 2026, a single-storey rear extension is broadly £2,000–£3,000 per square metre for a decent standard finish, so a 20 square metre kitchen extension lands somewhere around £40,000–£60,000. Two storeys costs less per square metre because the expensive parts — foundations and roof — are shared. Glazing, groundworks and a sloping site move it most.
What is the Party Wall Act and does it apply to me?
If you are excavating within 3 metres of a neighbour's structure and deeper than their foundations, or building on or against the boundary line, you must serve notice under the Party Wall etc. Act 1996 — usually two months before work starts. It is not planning permission and it is not optional. Serving it late is one of the most common and most expensive delays on an extension.
Do you build the whole extension?
No — we do the carpentry. Structural timber, floor joists, roof carpentry, studwork and first fix, then doors, skirting, architrave, staircases and finishing joinery on the second. You will want a main builder for the groundworks, brickwork and roofing, and we work alongside them.
Talk to us about the carpentry
Whether you have drawings already or are still working out what is possible, Alex will look at the carpentry and price it honestly.