The Core Difference Between Dormer and Velux Loft Conversions
A Velux loft conversion — sometimes called a rooflight conversion — adds roof windows into the existing roof slope without changing the roof structure. It is the simplest form of loft conversion: you are working within the existing envelope of the roof rather than expanding it. The space you gain is limited by the existing headroom under the roof ridge.
A dormer loft conversion adds a structural extension projecting from the roof slope, creating a box-shaped addition that provides additional headroom and floor space. The most common form in London and the South East is the rear dormer, which runs across the rear slope of the roof and creates a near-flat ceiling area across the full width or most of the width of the original roof.
Headroom and Usable Space
Headroom is the critical factor. In a Velux conversion, the usable floor area is limited to the section of the loft where the ridge height provides adequate standing height — typically two metres or more above the floor. Towards the eaves, the ceiling drops and the space becomes unusable for most purposes. On a typical two-storey London terrace, this often means a Velux conversion produces a relatively modest room, sometimes suitable for one bedroom but not generous.
A dormer dramatically increases the volume of usable space. The vertical rear wall of the dormer replaces the sloping rear roof face, giving full standing height across that portion of the floor. This makes it possible to create a substantially larger bedroom, often with an en-suite bathroom included, which would not be feasible in a Velux-only scheme.
Planning Permission Requirements
Velux conversions are almost always permitted development — they typically do not require planning permission because they do not exceed permitted development thresholds for roof alterations. This makes them simpler and faster to initiate, with no planning risk and no application fees.
Rear dormers on properties in Greater London fall under the Town and Country Planning (General Permitted Development) (England) Order for extensions and are often permitted development, subject to conditions including the dormer not exceeding the height of the original roof, being set back from the eaves and gable ends, and using materials that are similar in appearance to the existing house. However, in conservation areas, Article 4 directions, or for listed buildings, planning permission is required even for rear dormers. Always check the specific constraints applying to your property.
Certain dormer configurations — such as hip-to-gable conversions on semi-detached properties, or L-shaped dormers wrapping two slopes — may exceed permitted development limits and require planning permission regardless of location.
Building Regulations
Both types of conversion require building regulations approval. The structural works, floor upgrade, fire safety measures, insulation, staircase, and means of escape must all meet current standards. On a dormer conversion, the structural work is more extensive — roof timbers must be cut back, temporary support provided, and the dormer frame constructed correctly. Structural engineer input is usually required for the calculations.
Which Conversion Type Adds More Value?
Generally speaking, a dormer loft conversion that creates a full bedroom with en-suite adds more value to a property than a Velux conversion because it creates significantly more usable space. Estate agents typically value loft conversions based on the floor area and bedroom count they add. A dormer conversion creating an additional double bedroom and bathroom can add meaningfully to a property's value, particularly in London where bedroom count has a strong effect on price.
A Velux conversion may add less value but costs less to build — making the value-to-cost ratio potentially similar in some scenarios. The right choice depends on the existing loft dimensions, the intended use, planning constraints, and budget.
Making the Right Choice for Your Property
At RCB Design and Build, we assess both the loft geometry and the planning position before recommending a conversion type. Many properties can technically accommodate both options, and the right choice depends on your goals and constraints. We carry out a pre-design review covering roof structure, head height, planning constraints, and intended use, then present options with a realistic indication of scope and cost. Contact us to arrange a review of your loft.