Loft conversion cost estimator
Get a ballpark cost for your loft conversion based on the type of conversion, size and finish level. Includes a payback period based on estimated property value uplift, a cost breakdown by trade, and planning permission guidance for each type. UK figures.
A loft conversion is one of the best-value home improvements you can make, but only if your existing loft has sufficient headroom. As a rule of thumb you need at least 2.2 metres from the floor joists to the ridge beam. Measure before you commit to anything. If you are below this threshold a mansard or significant structural intervention will be needed, which changes the cost considerably.
Check headroom first
You need at least 2.2m from floor joists to ridge beam for a comfortable conversion. Below 2m and a Velux-only conversion may not be viable. Measure before calling any architect or builder.
Floor strengthening
Loft floors are usually designed for storage, not habitation. Converting to a bedroom almost always requires new structural floor joists. This is included in the build cost estimates above but worth confirming with your builder.
Adding a bedroom adds most value
A loft that creates a fourth bedroom from a three-bedroom home typically adds 10 to 20% to property value. The jump from two to three bedrooms is similarly significant. Check comparable sold prices on Rightmove to estimate your specific uplift.
Fire safety requirements
Building regulations require a protected escape route from a loft bedroom. This usually means fire doors and sometimes upgrading the staircase. Your building inspector will advise, but factor in potential additional cost at the design stage.
Please note: All figures are ballpark estimates based on typical UK build costs in 2025 and are for budgeting guidance only. Actual costs vary significantly by location, contractor, site conditions and specification. Property value uplifts are indicative estimates based on general market patterns and will vary by area and property type. Always obtain at least three itemised quotes and seek a professional valuation before proceeding. This is not financial advice.
What does the loft conversion cost calculator cover?
The loft conversion cost calculator estimates the total project cost based on the conversion type, floor area and region. It covers three main types:
Velux (rooflight) conversion: The simplest and cheapest type — no changes to the roofline, just installation of roof windows and internal fit-out. Suitable only where the loft has sufficient head height.
Dormer conversion: A structural extension of the roof to create additional headroom and floor space – typically a rear dormer. The most common type of terraced and semi-detached houses.
Hip-to-gable conversion: Extends the sloping hip end of the roof to create a vertical gable wall – often combined with a rear dormer. Suitable for semi-detached or detached properties where the roof slopes at the end.
How to use this calculator
Conversion type: Select the type that best describes your project. If you are unsure which is appropriate for your property, an architect or loft conversion specialist can advise – most offer free initial consultations.
Number of bedrooms added: The number of rooms created in the conversion. A single bedroom with en-suite is standard; some conversions create two smaller rooms.
Region: Costs vary significantly by location.
Reading the result: The calculator shows a cost range including structural work, insulation, flooring, electrics, plumbing (if a bathroom is included), staircase, and finish – plus architect fees and building regulations. Excludes planning permission if required (though many loft conversions fall under permitted development).
Frequently asked questions
Does a loft conversion add significant value? A loft conversion adding a bedroom with an en-suite typically adds 10 to 20% to a property’s value, depending on the local market and the existing number of bedrooms. In areas where four-bedroom properties command a significant premium over three-bedroom ones, the ROI can be very strong.
Do I need planning permission for a loft conversion? Most loft conversions in houses fall under permitted development and do not require planning permission, subject to volume limits and height restrictions. Conversions on listed buildings, in conservation areas, or on flats always require planning permission. Check with your local planning authority.
How long does a loft conversion take? A standard dormer conversion typically takes six to ten weeks on-site. The property remains liveable throughout in most cases.
The information on this site is for educational and informational purposes only. Nothing on Be More Peter constitutes financial advice, and nothing should be treated as such. The calculators, articles and tools on this site are designed to help you understand financial concepts and run indicative numbers - they are not a substitute for professional advice tailored to your specific circumstances. Before making any financial decision - including decisions about mortgages, pensions, investments, savings or insurance — you should seek independent financial advice from a qualified adviser who is authorised and regulated by the Financial Conduct Authority (FCA). Calculator results are estimates based on the information you enter and the assumptions built into the tool. They are illustrative only and should not be relied upon as accurate projections. Rates, thresholds and rules change regularly - always verify figures with an appropriate professional or official source before acting on them. Be More Peter is not authorised or regulated by the FCA. The content on this site does not constitute a financial promotion.
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