A basement conversion turns an existing cellar or below-ground space into a habitable room, or excavates a new basement beneath an existing home. In Cornwall, this typically means dealing with granite bedrock, high water tables on coastal sites, or clay soils — all of which shape the waterproofing strategy and cost. Converting an existing cellar without extending the footprint is often permitted development; excavating a new basement usually requires planning permission and a more involved structural process.

A loft conversion transforms unused roof space into a bedroom, office or bathroom, typically adding a dormer or roof lights for headroom and natural light. It's generally less structurally complex than a basement conversion — no excavation, no underpinning — and often falls within permitted development, provided the extension stays within volume limits and doesn't affect the roof line visible from the street. Loft conversions in Cornwall still need to account for insulation and ventilation carefully, given the exposure many properties face to coastal weather.

Loft conversions are generally the more affordable route per square metre, since there's no excavation or underpinning involved. Basement conversions cost more, particularly where ground conditions demand specialist waterproofing or where new excavation is required rather than converting existing below-ground space. Disruption — A loft conversion is usually less disruptive to daily life, since work is contained to the roof space and doesn't affect the ground floor. A basement conversion, especially a new excavation, is a significant structural project that can affect access and services throughout the build.

Suitability — Loft conversions depend on adequate head height and roof pitch; not every property has the roof structure to support one. Basement conversions depend on ground conditions and water table — not every site is straightforward, and a survey is essential before committing to a plan.

Added value — Both typically add meaningful value by creating usable space without extending the building's footprint, which matters particularly on sites where planning constraints limit above-ground extension. Which adds more value depends on the property, the local market and how the finished space is used.

The right choice often comes down to what your property allows rather than preference alone. A steep, tall roof with good head height points toward a loft conversion as the more straightforward route. A property with an existing cellar, or where roof space is too restricted to convert, points toward a basement. Some properties in Cornwall — particularly older cottages with low roof pitches and no existing cellar — may find neither route straightforward, in which case an extension might be worth considering alongside both options. A site survey is the only reliable way to know which route works for your home.