How Much Does a Loft Conversion Cost? A 2026 Surrey Guide

What decides the cost of a loft conversion?

The biggest single factor is the type of conversion your roof can take, and the four types are genuinely different pieces of work:

Which type your roof can take is not a free choice: it is set by the existing roof structure, the headroom and the planning position on your street. Alongside the type, these are the drivers, with a general sense of how much each one tends to move the total:

Relative weight is a general guide from the lofts we build, not a rule. On some roofs the structure alone outweighs everything else, which is why we confirm what is achievable before putting a figure in writing.

What makes a loft conversion cost more?

• A trussed roof. A traditional cut roof is straightforward to open up. A modern trussed roof has to be structurally reinforced or replaced as the space is created, which adds steel, engineering and labour.
• Too little headroom. Where there is not enough height under the ridge, the options are to raise the ridge or lower the ceilings in the rooms below. Both are achievable and both add cost.
• A difficult staircase route. Fitting a compliant staircase is easy in some homes and a genuine design puzzle in others. A complex route adds both cost and build time.
• Larger dormers and more glazing. A large dormer, a Juliet balcony or a run of rooflights adds structure and glazing compared with a plain rooflight conversion.
• An en-suite. A shower room or bathroom on the new floor means first- and second-fix plumbing, waterproofing and drainage, priced as its own line.
• Specification and location. The standard of finish is where two identical lofts diverge, and projects in South West London typically run a little above their Surrey equivalents.

How long does a loft conversion take?

A standard loft conversion typically takes 6 to 10 weeks from start to completion, and a mansard conversion 10 to 14 weeks. Most of the structural work is carried out from scaffolding, so daily life continues largely as normal. The structural engineer's calculations, the drawings, the building regulations submission and any planning application come first, and we agree the programme with you before work begins.

Do I need planning permission for a loft conversion?

Many do not, but it depends on the property. Many loft conversions fall within Permitted Development and do not need a full planning application, subject to volume limits (commonly 40 cubic metres for terraced houses and 50 for semi-detached and detached homes) and other conditions. Alterations to a roof slope facing the road, homes in conservation areas, listed buildings and flats are treated differently. Building Regulations approval is always required, whatever the planning route. Our guide to planning permission for a loft conversion in Surrey sets out the rules in more detail, and we confirm the position for your property as part of every Cost Proposal.

What should I budget for beyond the build?

Some costs sit outside any builder's figure, or are easy to miss when comparing proposals. Check whether each of these applies to you:

• A planning application or lawful development certificate fee, where one is needed
• Party wall surveyor fees, where the new steels bear on a wall shared with a neighbour
• Clearing the loft, and any temporary storage for what it held
• Furniture, blinds and fitted storage for the new rooms, if they are not in the proposal
• A contingency for what an older roof reveals once it is opened up

It is also worth checking whether each figure you are given includes VAT.

What should a complete loft conversion proposal include?

When you compare proposals, check that each of these is priced rather than assumed:

• A structural assessment and an engineer's calculations for the steels, joists and beams
• A made-to-measure staircase that meets building regulations
• The dormer, mansard or hip-to-gable build itself, weathertight, with matched external finishes
• High-performance insulation and the rooflights or dormer windows
• First- and second-fix plumbing and electrics, including any en-suite
• Plastering, flooring and full decoration
• Building control inspections and the completion certificate

The last line is the one most often missing from a cheap-looking figure. The completion certificate is what a future buyer's solicitor will ask for, so it matters as much as the build itself.

How do I compare loft conversion proposals fairly?

A lower number is not automatically better value, and the gap between two proposals is usually scope rather than margin. Three questions expose most of it:

1. Is it itemised, or a single figure? A single figure cannot be interrogated, and cannot be adjusted without renegotiating the whole job.
2. What is excluded? Read the exclusions before the inclusions. Building control fees, staircase joinery and decoration are common omissions.
3. What happens if the roof is not as expected? Older roofs hold surprises. A good proposal says in advance how a variation is priced and agreed.

How do we price a loft conversion?

We design and build under one roof, which means the person pricing the work is the person who will carry it out. After a free site visit to assess your loft, we produce a fully itemised Cost Proposal, line by line, with the exclusions stated plainly. Our weekly billing model means you pay for completed, signed-off work each week rather than handing over a large deposit in advance, so the cost stays visible as the project runs rather than arriving as a surprise at the end.

Our loft conversions service page explains how we work, and our Knowledge Centre answers the most common questions on timescale and process. If you are still deciding whether to buy a house for its loft, our Pre-Purchase Consultation gives you a builder's written budget before you commit. Otherwise, a free Cost Proposal is the next step.