What changes between the main types?
A rooflight conversion largely works within the existing roof form. That can avoid the external enlargement involved in other options, but the habitable floor, stair, insulation and fire precautions still need to be provided. Retaining the roof outline does not mean that only windows and decoration are needed. [1] [2]
A dormer creates additional usable space through a projecting roof structure. Its cost scope includes the altered roof support and the new walls, roof and junctions. An L-shaped arrangement adds linked roof enlargement; its particular size and construction need pricing from the drawings. [1] [4]
Hip-to-gable and mansard options change the roof form more extensively. Ask the quotation to identify the actual rebuilding, weatherproofing and structural work rather than apply an assumed percentage premium to another project's price. The conversion type is a starting point for the scope, not a fixed tariff. [1] [4]
The work below the loft can be significant
Include the new stair and its opening, changes to the landing and any work needed to support the new loads below. The fire strategy may require alterations through the existing house, not only doors or alarms in the new loft. [5] [6] [7]
An en-suite adds drainage, water and ventilation work. Its location affects how those services can be connected, so do not treat the sanitary fittings allowance as the complete bathroom cost. Ask the designer and contractor to identify the full installation and making-good scope. [8] [9] [10]
Separate the budget into clear packages
For comparison, ask each quotation to distinguish:
- Surveys, design and structural information.
- Applications and building control charges where required.
- The complete construction scope, including work to existing rooms.
- Sanitary fittings, finishes and other items you select.
- Excluded work, unresolved allowances and VAT. [2] [11] [12]
Where party wall procedures or ecological work are needed, allow for them separately unless expressly included. Party wall surveyors' fees are not nationally fixed, and ecological requirements depend on the site's evidence and proposed impact. [13] [14]
Check VAT and exclusions
HMRC states that most building work to houses and flats is standard-rated at 20%, although specific exceptions exist. Do not assume that calling an existing-house loft project a 'conversion' automatically qualifies it for a reduced rate. Check the treatment of the actual work and whether the quoted total includes VAT. [12]
Ask what happens if an existing support or other hidden condition differs from the design assumptions. Agree how any change will be described and priced before it proceeds. A lower initial figure is not a like-for-like comparison where essential work remains unspecified. [6]
For work affected by the changes from 24 March 2027, check the applicable energy and ventilation specification before finalising prices. Transitional arrangements mean the correct edition depends on the project's circumstances, not simply the quotation date. [15] [16] [17]
The useful budget is the total for your documented scheme. Avoid adding an unsupported London cost-per-square-metre figure to a roof area that may not represent usable finished accommodation.