A loft conversion in London costs £18,000–£95,000+ depending on the type and specification. A standard dormer conversion — the most popular type — costs £35,000–£65,000 in London at mid-range specification, before VAT and professional fees. London costs run approximately 20–30% above the national average. The conversion type (Velux, dormer, mansard, hip-to-gable) is the single biggest cost variable.
Loft Conversion Costs by Type (London, 2025)
The type of loft conversion is the primary cost driver. Your roof structure, planning constraints, and the amount of space you want to create will largely determine which type is suitable for your property.
No structural changes to the roofline. Velux windows inserted into the existing roof slope.
A box-shaped extension built out from the existing roof slope, creating vertical walls and a flat or pitched roof.
The sloping hip end of the roof is replaced with a vertical gable wall, extending the ridge and creating more internal space.
The entire roof structure is rebuilt with near-vertical (72°) rear walls and a flat top. Maximises space but is the most complex type.
Two dormers joined at a right angle — one on the rear slope and one on the back addition roof. Common in Victorian and Edwardian terraces.
* All costs exclude VAT (20%), professional fees, and finishes. Based on standard to mid-range specification.
Loft Conversion Costs by Specification Level
For a standard dormer conversion in London, the specification level has as much impact on the final cost as the conversion type itself. The table below shows what each level includes and the typical price range.
- Standard staircase (straight or space-saver)
- Budget-grade insulation (meets minimum Building Regs)
- Basic electrics (lighting, sockets, smoke alarm)
- Velux windows or standard uPVC dormer windows
- Plasterboard walls and ceilings, emulsion finish
- No en-suite bathroom
- Feature staircase with glass balustrade or oak handrail
- Enhanced insulation (above Building Regs minimum)
- Full electrical installation with USB sockets
- Aluminium-framed dormer windows or Velux roof windows
- En-suite shower room with quality sanitaryware
- Fitted wardrobes in eaves storage areas
- Bespoke staircase with architectural detailing
- Structural glazing, Juliet balcony, or roof terrace
- Underfloor heating throughout
- Full bathroom with freestanding bath and walk-in shower
- Bespoke joinery and fitted furniture
- Air conditioning or MVHR ventilation system
What Affects the Cost of a Loft Conversion?
Beyond the conversion type and specification level, several other factors influence the final cost. Understanding these helps you make informed decisions about where to invest and where to economise.
The type of conversion is the biggest single cost variable. A Velux conversion costs roughly half the price of a mansard. The type is largely determined by your roof structure and planning constraints.
The gap between a basic and premium specification can be £40,000 or more on the same conversion. En-suite bathrooms, bespoke joinery, structural glazing, and underfloor heating all add significant cost.
Removing chimney breasts, installing new steel beams, strengthening floor joists, or working on a property with an unusual roof structure all add cost. A structural engineer's report is required for any structural alterations.
Labour costs in London are 15–25% higher than the national average. Central and West London builders charge more than East London and Hertfordshire builders for equivalent work.
Scaffolding in a terrace with a narrow access road, ULEZ charges, parking restrictions, and skip permits all add to the cost of a London loft conversion.
A Certificate of Lawful Development costs £258. Full planning permission costs £258 for a householder application. Architect fees for a loft conversion typically run £2,000–£5,000.
Cost Breakdown: Labour, Materials & Professional Fees
For a typical dormer loft conversion in East London at mid-range specification (total build cost approximately £45,000–£60,000 before VAT), the cost breakdown typically looks like this:
Carpentry (roof structure, dormer frame, staircase), roofing, plastering, electrics, plumbing (if en-suite), decoration
Timber, roofing felt, tiles, insulation, plasterboard, windows, doors, sanitaryware (if en-suite), flooring
Design, planning drawings (if required), Building Regulations drawings, contract administration
Structural calculations, steel beam specification, floor joist strengthening design
Certificate of Lawful Development (£258), Building Regulations Full Plans application (£400–£800)
Site management, scaffolding, skip hire, ULEZ/parking permits, insurance, plant hire
Does a Loft Conversion Add Value to Your Home?
A well-executed loft conversion is one of the highest-ROI home improvements available to London homeowners. Adding a bedroom and bathroom to a property increases its value not just by the cost of the works, but by moving it into a higher price bracket — a 2-bed becomes a 3-bed, or a 3-bed becomes a 4-bed.
The value added depends on the area, the quality of the conversion, and the bedroom count of comparable properties nearby. The table below shows indicative figures for the areas TCM works in.
| Area | Estimated Value Added | Typical ROI | Notes |
|---|---|---|---|
| London (Inner) | £75,000–£150,000 | 120–180% | Highest ROI in the UK. Space is at a premium. A well-executed loft conversion in Inner London almost always adds more value than it costs. |
| East London (Ilford, Redbridge) | £40,000–£90,000 | 90–140% | Strong ROI. Demand for 3–4 bed properties is high. A master bedroom suite with en-suite is the most valued addition. |
| Hertfordshire | £30,000–£70,000 | 80–120% | Good ROI, particularly in commuter towns. Family homes with 4+ bedrooms command a significant premium. |
| North London (Barnet, Enfield) | £50,000–£110,000 | 100–160% | Strong ROI. Victorian and Edwardian terraces with L-shaped dormers are particularly sought after. |
* Value added figures are indicative estimates based on Rightmove and Land Registry data. Actual figures vary by property type, condition, and local market conditions.
How to Get an Accurate Quote for Your Loft Conversion
Online cost guides give you a useful starting range, but the only way to get an accurate price for your specific loft conversion is to have a builder visit your property. The roof structure, head height, access, and your specification requirements all affect the cost in ways that cannot be assessed remotely.
Always request an itemised quotation — not a single lump sum. A detailed quote allows you to compare quotes meaningfully, understand exactly what is included, and identify where costs can be adjusted if the initial figure is above budget.
Ask specifically whether the quote includes VAT, scaffolding, skip hire, Building Regulations fees, and the staircase. These are the items most commonly omitted from initial quotes. Get at least three quotes and check that each builder can provide references for completed loft conversions.
TCM Building & Maintenance provides free site surveys and detailed, itemised quotations for loft conversions across London and Hertfordshire. We have completed over 200 loft conversions in the last 15 years. No obligation. No vague estimates.