Permitted Development Value Calculator UK

Yes, in most cases extensions add meaningful value, but the amount varies significantly by extension type, quality of finish, location, and current property size. Loft conversions and kitchen extensions tend to deliver the strongest returns — often 5–15% added value. However, over-improving a property beyond the ceiling price for the street can mean the build cost exceeds the value gain.

A loft conversion typically adds 10–15% to a property's value, making it one of the best-return home improvements available. On a £350,000 home, that is £35,000–£52,500 in added value. Build costs range from £25,000 for a basic conversion to £60,000+ for a dormer or mansard. ROI is typically 50–150% depending on specification and location.

Under permitted development rights (PD), you can typically add a single-storey rear extension up to 4m (detached) or 3m (attached) deep, a loft conversion up to 40m³ (terraced) or 50m³ (detached/semi), a side extension up to half the width of the original house, and an outbuilding up to 2.5m high within the garden. These rights do not apply in Conservation Areas, AONBs, or if your property has an Article 4 direction.

The 4m rule refers to the maximum rear extension depth for detached houses under permitted development — you can extend up to 4m deep without planning permission. Attached (terraced and semi-detached) houses are limited to 3m. Under the Larger Home Extension scheme (prior approval), detached houses can extend up to 8m and attached houses up to 6m, subject to neighbour consultation.

A garden office (outbuilding) generally adds modest market value — typically 1–5% — since buyers don't always factor in a garden office as strongly as internal living space. However, a well-built garden office can significantly increase saleability and appeal to remote workers. Build costs range from £8,000 for a basic timber structure to £30,000+ for an insulated, fully wired, purpose-built office.

Larger extensions don't always proportionally increase value. Typically, extensions that create a new bedroom or significantly enhance a kitchen-diner deliver the highest value uplift per pound spent. Very large extensions on modest properties risk over-developing beyond the ceiling price for the local street. Always compare your projected post-extension value with the highest-priced comparable properties nearby.

Prior approval is a lighter-touch planning process required for certain permitted development works, including the Larger Home Extension scheme (extending beyond standard PD depths), loft conversions in some cases, and changes of use. You submit a prior approval application to the local planning authority, which has 42 days to object. If it doesn't object, you can proceed. It is not full planning permission but checks specific impacts such as flooding, transport, and neighbour amenity.

Yes, in most cases. A loft conversion to create a bedroom is permitted development for houses (not flats) provided the roof space added does not exceed 40m³ (terraced) or 50m³ (detached/semi-detached), no part of the extension exceeds the highest part of the existing roof, no raised platform/balcony is created, and materials are similar in appearance to the existing house. Building regulations approval is still required regardless of PD status.

An Article 4 direction is a measure used by local planning authorities to remove some or all permitted development rights in a specific area — typically Conservation Areas, National Parks, or areas of special character. If your property is subject to an Article 4 direction, you may need full planning permission for works that would otherwise be permitted development. Check with your local council before starting any works.

A single-storey rear extension typically adds 3–10% to a property's value, depending on what it creates. If it extends a kitchen into a full kitchen-diner — particularly popular with family homes — the value uplift can be at the higher end. If it simply enlarges a room without adding a bedroom count, it is usually at the lower end. Build costs typically range from £1,500–£2,500/m² depending on specification and location.

For a single-storey rear extension on a detached house: up to 4m deep, not higher than 4m, and not wider than the original house. Under the Larger Home Extension prior approval scheme, up to 8m deep. For an attached house, standard PD allows 3m (prior approval up to 6m). Side extensions are limited to half the width of the original house and must be single storey (max 4m high). These limits apply only where permitted development rights are intact.

No. Permitted development rights are attached to the property, not the owner. If a previous owner has already used their PD allocation (e.g. built a rear extension), the new owner cannot automatically use PD for another rear extension of the same type. The PD calculation looks at what has been built since the original house was constructed (typically the 1948 baseline). You would need to apply for full planning permission for any further extension beyond the PD limits already used.