Loft Conversion Cost Calculator: Get Your 2026 Estimate in 60 Seconds

The calculator starts with a base rate for your chosen conversion type, then adjusts for floor area, property type, an ensuite and finish level. Those base rates come from itemised quotes and final accounts on loft conversions we've built across Oxford and Oxfordshire over recent years, updated for 2026 material and labour costs.

Thorncliffe Road Case Study

A premium dormer loft conversion featuring a Juliette balcony, oak staircase lines, and ensuite shower room, built to strict local planning rules in Summertown, Oxford.

The biggest single factor is the conversion type. A Velux conversion keeps the existing roofline, so it's the cheapest route. A dormer adds headroom and floor space by extending out from the roof slope, which is why it's the most popular choice on Victorian and Edwardian terraces. Hip-to-gable conversions suit semis and end-of-terraces by building up the sloped side of the roof, while a mansard rebuilds almost the entire roof structure — maximum space, maximum cost.

What the calculator can't see is your actual roof. Head height, the position of the existing staircase, the condition of the roof timbers and whether steel beams are needed all shift the final figure, which is why we always confirm pricing with a free site visit and structural calculations before quoting.

The figures cover the full build: structural work and steels, insulation to current Building Regulations standards, the new staircase, windows or dormers, first and second fix electrics and plumbing, plastering and decoration, plus building control sign-off. They also allow for the paperwork most homeowners forget to budget for — structural calculations, Building Regulations drawings and, where you share a wall with a neighbour, notices under the Party Wall etc. Act 1996. We handle all of that in-house.

Comparing Loft Conversion Styles

Each loft type dictates the total volume gained. Rooflights (Velux) preserve local slates, while Dormer flat roofs construct vertical space, and Hip-to-Gable vertical profiles accommodate standard staircase transitions.

Most loft conversions in England fall under Permitted Development, so full planning permission often isn't needed. The main exceptions are flats, listed buildings and homes in conservation areas covered by an Article 4 Direction — common in North Oxford, Jericho and parts of Headington — where an application to Oxford City Council (or Cherwell, West Oxfordshire, South Oxfordshire or Vale of White Horse District Councils outside the city) is usually required. If that applies to you, we'll flag it at the site visit and manage the application.

Treat the result as a realistic budgeting range, not a quote. In our experience, a well-built calculator lands within 10–15% of the final figure for a straightforward loft — close enough to decide whether the project stacks up. For example, our dormer loft conversion on dormer loft conversion on Thorncliffe Road in Summertown, complete with a Juliette balcony and ensuite, came in comfortably within the range this calculator would have predicted for a dormer of that size with a bathroom added.

Attic Structural Engineering

Structural steel joist support beams being positioned manually on a Victorian terraced home conversion. Pre-planning steel placements prevents deflection in structural ceiling zones below.

Where estimates drift is on the things only a survey reveals: shallow roof pitches that need restructuring, chimney removals, or older properties — like the Victorian terraces we've converted on Leckford Road and James Street — where the roof structure needs more steel than a modern home would.

Use it to sanity-check your budget, then get it firmed up. As an FMB-listed, TrustMark-approved builder, we provide itemised fixed-price quotes rather than vague estimates, with staged payments tied to completed milestones and a 10-year workmanship guarantee. Every project has a named project manager — James Crawford, our Project Director — so you always know who's accountable.

Once you've run the loft conversion cost calculator, the next step is simple: book a free site consultation and we'll turn your estimate into an exact, itemised fixed-price quote. Call us on +44 1865 554333 or email office@crawfordconstruction.co.uk.

Note: single-storey extensions run at £2,800–£3,500 per square metre in Oxford, so a loft conversion frequently delivers a similar amount of usable space for a comparable or lower total.