Loft Conversion Costs in Cambridge and Cambridgeshire
A loft conversion in Cambridgeshire generally falls between £22,000 and £80,000 including VAT, and where a project lands inside that range depends far more on the type of conversion than on the floor area.
This guide sets out the published 2026 cost benchmarks by conversion type, the head-height check that decides what is possible, the building regulations that always apply, and the costs that sit outside a builder’s price. It is general budgeting information rather than a quotation.
On this page · about an 8-minute read · reviewed by Daryl Edwards, Company Director · last reviewed 31 August 2026
- Loft conversion costs at a glance
- A simple way to form an early loft budget
- Why the published ranges disagree
- Head height: check this before anything else
- Building regulations always apply
- Planning permission: usually not needed, but check
- Costs to allow for outside the builder’s price
- Will a loft conversion add value?
- How to get a useful loft conversion quotation
- Relevant loft projects
- Loft conversion cost questions
- How this guide was prepared
Loft conversion costs at a glance
The main difference between a loft and an extension is what drives the budget. With an extension, square metres do most of the work. With a loft, the question is how much of the existing roof has to be rebuilt to make the space usable, and that is a step change rather than a gradual increase.
| Conversion type | Typical cost per m² | Typical project cost |
|---|---|---|
| Rooflight (Velux) | around £920 | £22,500–£30,000 |
| Dormer | around £1,670 | £40,000–£60,000 |
| Hip-to-gable | around £2,000 | £50,000–£70,000 |
| Mansard | around £2,170 | £50,000–£80,000 |
Rates and project ranges are from Checkatrade’s loft conversion cost guide, updated March 2026, and exclude VAT. A second 2026 source, Homebuilding & Renovating, quotes higher all-in figures — £55,000 to £80,000 for a standard rear dormer with a staircase — which is dealt with below.
A simple way to form an early loft budget
Take the type of conversion your roof will realistically take, multiply the finished floor area by the rate above, and add 20% VAT. Loft conversions to an existing home are standard-rated, so the VAT is real money and belongs in the first number you write down rather than the last.
Worked example — 25m² rear dormer. At £1,670 per m², that is £41,750 before VAT, or £50,100 with VAT at 20%. Set against Homebuilding’s £55,000 to £80,000 for the same job, a sensible planning band for a typical Cambridgeshire rear dormer is £45,000 to £70,000 including VAT.
Worked example — 20m² rooflight conversion. At £920 per m², that is £18,400 before VAT, or £22,080 with VAT. Homebuilding puts a small bedroom-only rooflight conversion at “from around £25,000”. Planning band: £22,000 to £32,000 including VAT.
Neither example includes the costs listed further down that sit outside a builder’s price, and neither includes a bathroom.
Why the published ranges disagree
They are measuring different things, and it is worth understanding why before relying on any of them.
A per-square-metre rate is a construction rate: it answers what it costs to build the thing. An all-in project figure usually includes the staircase and the finishes, sometimes the design fees, and sometimes an assumption about a shower room. A £41,750 dormer and a £60,000 dormer can be the same roof.
The second reason is definition. “Dormer conversion” covers a modest box on a rear slope and a full-width dormer with three windows and a bathroom. Nobody publishing a national average can tell you which one you are getting.
The useful comparison is therefore not between guides but between quotations for a defined scope, which is why an itemised breakdown matters more than a single headline number.
Head height: check this before anything else
The one thing that can rule a loft conversion out is height, and you can check it yourself in ten minutes.
Measure from the top of the existing ceiling joists to the underside of the ridge. As a working rule you want around 2.4m to have comfortable options, because height is lost at both ends: the new floor structure sits above the old joists, and insulation plus plasterboard comes off the underside of the rafters.
Building regulations require 2m of headroom over the staircase, measured vertically from the pitch line of the flight. Where that is genuinely impossible, building control may accept 1.9m at the centre of the stair reducing to 1.8m at the worst point, but that is a concession to be argued for rather than a starting assumption.
With less than about 2.2m to the ridge, the usable options are lowering the ceilings in the rooms below, or a mansard or roof-raise that rebuilds the roof structure. Both are substantial projects with their own budgets, which is why head height is the first measurement we take on a site visit.
Building regulations always apply
Unlike some extensions, there is no version of a loft conversion that avoids building control. Turning roof space into a habitable room is a material change of use, and approval covers structure, fire safety, stairs, insulation and ventilation.
- Structure. Existing ceiling joists are almost never sized to be floor joists. New floor members are designed by a structural engineer and the calculations go to building control.
- Fire escape. A loft conversion in a two-storey house creates a third storey, and the staircase becomes a protected escape route to a final exit. In practice that means 30-minute fire-resisting construction to the new floor and to walls onto the stair, fire doors to rooms opening onto it, and mains-powered interlinked smoke alarms on every floor.
- Escape windows. Where the ground floor is open-plan, escape windows are needed with a clear opening of at least 0.33m², no dimension under 450mm, and a cill no more than 1.1m above floor level.
- Insulation. Current Part L targets apply to the new roof build-up. Your building control officer will confirm the U-values for the construction being used, which is worth agreeing early because it affects how much height the build-up takes.
None of this is optional, and all of it belongs in a properly scoped price. A quotation that is materially cheaper than the others has often left some of it out.
Planning permission: usually not needed, but check
Most loft conversions are permitted development, which is why the planning line in a loft budget is usually small or zero. The limits that matter are the volume allowance — 40m³ for a terraced house and 50m³ for a detached or semi-detached house — no extension beyond the existing roof plane on the principal elevation, and nothing above the existing ridge height.
Permitted development rights are removed or restricted in some places. Conservation areas, flats and maisonettes, and properties affected by an Article 4 direction all need checking before relying on them, and parts of Cambridge and several surrounding villages are affected.
Where the work is permitted development, a Lawful Development Certificate is usually worth the fee. It costs £274 — half the £548 householder application fee under the fee schedule effective 1 April 2026 — and it is the document that answers the question at sale, which is exactly when nobody wants an argument. Our permitted development guide sets out the limits and the certificate process in full.
Costs to allow for outside the builder’s price
- Architectural drawings — roughly £1,500 to £3,500
- Structural engineer — roughly £500 to £1,200 and above, depending on complexity
- Lawful Development Certificate — £274, or a householder planning application at £548 where permission is needed
- Building control — charged by the local authority or an approved inspector; ask for the fee in writing at the start
- Party Wall etc. Act 1996 — terraced and semi-detached properties usually need notice served on the neighbour, and surveyor’s fees if an award is required
- Bathroom fit-out — commonly £5,000 to £15,000 and above, and often quoted separately
- VAT at 20% on the building work
- Contingency — at least 10%, and 15% is more realistic on a roof, because what sits above the ceiling is not fully known until it is opened up
That last point is not a disclaimer. Lofts hide more surprises than extensions do: undersized purlins, redundant water tanks, chimney breasts removed downstairs without proper support, and occasionally bats.
Will a loft conversion add value?
We do not provide property-valuation advice and we do not promise a particular increase in value. A local chartered surveyor or qualified valuer can advise what an additional bedroom is worth on your street, which is a more useful answer than a national percentage.
What can be said is that the cost per square metre of usable space is usually lower for a loft than for a ground-floor extension, because the roof, the foundations and the footprint already exist. That makes a loft efficient. It does not automatically make it the right answer.
How to get a useful loft conversion quotation
Ask for everything you would ask of any builder, plus three things specific to lofts:
- Which conversion type is being priced, and why that type suits your roof.
- Whether the structural design is included, and who is paying the engineer.
- What the staircase allowance is. A staircase can be a few hundred pounds for a straight flight or well over £10,000 for something bespoke in a tight space, and the word on its own tells you nothing.
Then the usual: a written scope, named exclusions, the VAT position, a programme, stage payments and how variations are handled. Our guide on what a builder’s quotation should include covers the full checklist.
Relevant loft projects
The Cherry Hinton Road extension and loft conversion combined a loft conversion with a rear extension on an occupied house in Cambridge. The project page records the planning route — a householder application granted in August 2024, followed by a non-material amendment the following March covering fenestration changes, solar and two additional loft rooflights — which is a fair illustration of how a real scheme develops between consent and completion.
Our loft conversion service page sets out how we scope, price and run this type of work.
Loft conversion cost questions
How much does a loft conversion cost in Cambridge?
Most fall between £22,000 and £80,000 including VAT. A rooflight conversion sits at the bottom of that range and a mansard at the top, with dormers — the most common choice — usually £45,000 to £70,000 including VAT for a typical rear dormer.
Is a loft conversion cheaper than an extension?
Per square metre of finished space, usually yes, because the footprint, foundations and roof already exist. Whether it is the right choice depends on what you need the space to do and how much head height you have.
Do I need planning permission for a loft conversion?
Usually not, as most are permitted development. Conservation areas, flats, and properties with an Article 4 direction are the common exceptions. Building regulations approval is always required regardless.
How long does a loft conversion take?
It depends on the type of conversion, the structural work involved and the approvals route, so we give you a programme with your quotation rather than a general figure. Design and approvals normally add a period before work starts.
Can I stay in the house during the work?
Usually yes. The most disruptive phase is forming the stair opening and breaking through the ceiling. We agree access, working hours and dust protection before starting.
Why is VAT charged on a loft conversion when new houses are zero-rated?
Because a loft conversion is work to an existing dwelling, which HMRC standard-rates at 20%. Building a new house from scratch is zero-rated. It is one of the larger differences between the two budgets, and we cover it in our new build cost guide.
How this guide was prepared
Cost benchmarks are taken from Checkatrade’s loft conversion cost guide (updated March 2026) and Homebuilding & Renovating’s loft conversion cost guide (updated August 2026), and are quoted as published rather than adjusted. Planning application fees are from the Planning Portal fee schedule effective 1 April 2026. Building regulations requirements are drawn from the Approved Documents and local authority building control guidance. Worked figures are transparent multiplication of a published benchmark and are a starting point for a conversation, not a quotation.
Get a clear next step for your loft
Tell us what you want to achieve, and send whatever you already have. Photos and drawings help, but they are not essential to start — plenty of projects begin with a description and a site visit. We aim to respond to every enquiry within one working day with the most useful next step. Request a Quote.
More on planning, costs and contracts in our building guides. Some homeowners spread the cost — our building project finance page explains the options and who provides them.
Last reviewed: 31 August 2026. This guide applies to England. Costs, planning rules, fees and building regulations change; check current official guidance and obtain appropriate professional advice for your property and project.