How Much Does a Loft Conversion Cost and How Much Can You Borrow?
The cost of a loft conversion varies significantly by type. A basic Velux conversion — where you add roof windows but do not alter the roof structure — typically costs £20,000 to £30,000 and is the most affordable option. A dormer conversion, which extends the roof to create additional headroom, runs from £30,000 to £50,000. A hip-to-gable conversion, popular on semi-detached properties, costs between £35,000 and £55,000. At the top end, a full mansard conversion can reach £45,000 to £65,000 and is most common in London and the South East.
When sizing a secured loan, lenders look at two things: the equity available in your property and your ability to afford the monthly repayments. Most secured loan lenders will advance up to 80 to 85 per cent combined loan-to-value (CLTV) — that is, your existing mortgage balance plus the new secured loan as a proportion of your property's current value. For example, if your home is worth £350,000 and your outstanding mortgage is £200,000, an 85% CLTV lender could advance up to £97,500 as a secured loan.
On a £40,000 secured loan over 15 years at a rate of 9%, monthly repayments would be approximately £406. Over ten years at the same rate, repayments rise to around £507 per month. Extending the term reduces the monthly cost but increases total interest paid, so it is worth modelling different term lengths when comparing quotes.
Lenders will also want evidence of the planned works — typically contractor quotes and, where applicable, planning permission or a permitted development confirmation. Having these documents ready before you apply will speed up the process and strengthen your application.
Planning Permission and Permitted Development for Loft Conversions
Most loft conversions in England fall under permitted development rights, meaning they do not require a full planning application provided they stay within set limits. The key rules are that the additional roof space must not exceed 40 cubic metres for terraced houses or 50 cubic metres for detached and semi-detached houses, the conversion must not extend beyond the highest point of the existing roof, and no side-facing windows at lower floor level are permitted without obscure glazing.
However, permitted development does not apply in all cases. If your property is in a conservation area, an Area of Outstanding Natural Beauty, or is a listed building, you will need full planning permission regardless of the size of the conversion. Properties in Article 4 Direction areas — which strip back permitted development rights in certain localities — also require planning consent. Your local planning authority's website will confirm whether your property is subject to any restrictions.
Even where permitted development applies, you must obtain a Lawful Development Certificate from your local authority to confirm the works are lawful. This certificate is important when you come to sell the property, as buyers' solicitors routinely request it. The application costs approximately £100 in England and takes around eight weeks. Building regulations approval is required separately for all structural loft conversions regardless of planning status.
Secured loan lenders will typically ask to see planning permission or a Lawful Development Certificate as part of their underwriting process. If your conversion is in the planning phase, some lenders will issue a mortgage offer in principle while permission is awaited, allowing you to proceed with contractor negotiations in parallel.
