Do you need planning permission for a mansard loft conversion in a Kingston conservation area?

Yes, and this is one of the clearest cases where planning permission is required. Mansard conversions fall outside permitted development everywhere, and conservation area status in Kingston adds a further layer of scrutiny. For this project, Rosace submitted the extension and the mansard as two separate full planning applications to reduce the risk of one holding back the other.

How long did this project take, from first design to planning approval?

From initial design through to planning approval, this project took around five months. Conservation area schemes with two separate applications generally take longer than a single straightforward submission, so this was a reasonably efficient outcome for the complexity involved.

What was the approximate cost of this project?

Combined, the single storey extension and mansard loft conversion came to around £245,000, split roughly £150,000 for the extension and £95,000 for the mansard. As with any conservation area project, cost reflects the additional detailing needed to satisfy planning requirements, particularly around the roof.

Can Rosace deliver the interior design as well as the architecture?

Yes, interior design is in-house at Rosace, designed alongside the architecture from the first sketch rather than added afterwards.

Mansard Loft Conversion Kingston, KT2

The brief

The client's goals were practical: a proper master suite, with ensuite and balcony, and a meaningful uplift in the property's value. A single storey rear extension was added to free up space at ground level, but the more significant move was upstairs.

Rosace's approach centred on maximising value from the roof. In place of a neglected attic, we designed a full mansard loft conversion, generous enough to accommodate the new master suite in full. The mansard's steep-pitched form also created the opportunity for a recessed balcony at roof level, giving the new bedroom direct outdoor space that a standard dormer conversion could not have offered. At the rear, the single storey extension was kept slim and long rather than broad, adding usable space without overwhelming the garden or the proportions of the house behind it.

Planning context

Because the property sits within a conservation area, this project required a full planning application to the Royal Borough of Kingston upon Thames, rather than any permitted development route. Rosace made the decision to submit the rear extension and the mansard loft conversion as two separate applications, a strategy that reduces the risk of one contentious element jeopardising approval for the whole scheme. The application was supported by a design and access statement, addressing how the proposals related to the conservation area's character.

Kingston, like most London boroughs, applies the 70-degree rule to mansard roofs, requiring a steep, consistent pitch on the principal slope, a constraint that shaped the mansard's proportions and its relationship to the roofline of neighbouring properties from the earliest sketches.

Design & materials

Structurally, the single storey extension relies on steel beam lintels to keep the rear elevation open, glazed in slimline aluminium sliding doors and finished with a liquid rubber flat roof for a clean, low-profile silhouette. The mansard above is faced in dark handmade clay tiles, a traditional material that reads correctly against Kingston's conservation area context, while the vertical elements of the roof are finished in painted white stucco rather than brick or tile, giving the new storey a lighter, more contemporary presence above the existing facade. Together, the two material palettes let the extension and the loft conversion read as distinct interventions rather than a single, over-worked addition.