Loft Conversion Cost per Square Metre in North London

Introduction

A loft conversion is typically the most cost-effective way to add a bedroom and bathroom to a north London Victorian or Edwardian house. The cost per square metre is lower than for a basement extension and comparable to or lower than a ground floor extension, and the value added in terms of bedroom category and sale price is consistently strong. This guide provides a detailed breakdown of loft conversion costs by type and specification, explaining the factors that drive cost variation and what is typically included in a contractor's loft conversion price in north London.

Loft Conversion Types and Cost

Rooflight (Velux) Conversion

The simplest and lowest-cost loft conversion — the existing roof structure is retained and VELUX or similar rooflights are inserted to provide daylight and ventilation. Only viable where the existing roof pitch provides adequate head height at ridge level (typically 2.3m+ to underside of ridge). Floor joists are reinforced to carry floor loading, a loft staircase is installed, and insulation, plastering and second-fix finishes are completed.

Parameter	Rooflight Conversion
Usable floor area (typical)	15–25 sqm
Construction cost per sqm	£1,800–£2,500/sqm
Total cost (20 sqm)	£36,000–£50,000
Planning	No planning permission required in most cases (PD)

Rear Dormer Conversion

A dormer on the rear roof slope — a flat-roofed box extension projecting from the main slope — provides maximum floor area and standing headroom. The most common loft conversion type for Victorian and Edwardian terraces in north London.

Parameter	Rear Dormer Conversion
Usable floor area (typical)	20–40 sqm
Construction cost per sqm	£2,200–£3,000/sqm
Total cost (30 sqm usable)	£66,000–£90,000
Planning	PD (rear slope, within volume limits); conservation area may require planning

Hip-to-Gable Conversion

Replacing the hipped end of the roof with a vertical gable wall, increasing the usable floor area at the sides of the loft. Often combined with a rear dormer for maximum floor area. Typically requires planning permission.

Parameter	Hip-to-Gable + Rear Dormer
Usable floor area (typical)	30–55 sqm
Construction cost per sqm	£2,500–£3,500/sqm
Total cost (40 sqm usable)	£100,000–£140,000
Planning	Planning permission typically required

Mansard Conversion

A mansard involves rebuilding both rear slopes and often the ridge line to create a near-vertical rear face with a shallow pitch — common in terraced houses and in conservation areas where a traditional mansard form is acceptable. Higher construction cost due to the extent of roof structure replacement.

Parameter	Mansard Conversion
Usable floor area (typical)	25–50 sqm
Construction cost per sqm	£3,000–£4,500/sqm
Total cost (35 sqm usable)	£105,000–£157,000
Planning	Planning permission required

What Drives Cost in a Loft Conversion

The main variables affecting loft conversion cost in north London are:

Staircase design: A straight flight staircase in a dedicated staircase void is the simplest and lowest cost. A space-saving staircase (alternating tread, storage stair) in a very constrained hall costs more for less performance. An elegant timber open-riser staircase with bespoke balustrade adds significant cost.
Bathroom specification: A simple shower room with ceramic tiles and standard sanitaryware costs £10,000–£15,000. A high-specification en-suite with stone tiles, bespoke vanity and premium fittings costs £20,000–£40,000.
Dormer roof specification: A flat EPDM-covered dormer is standard. A zinc-clad dormer face is a premium aesthetic option at higher cost. Lead flashing at dormer junction is standard in conservation areas.
Structural steelwork: Where the existing roof structure cannot be simply reinforced and new steels are required to create the open plan loft floor, structural steel cost adds £5,000–£20,000.
Access constraints: A loft conversion in a property with difficult access (narrow street, no rear access, restricted scaffolding) adds preliminary cost.

Typical Full Cost of a North London Dormer Loft Conversion

Element	Cost Range
Structural and roof works (dormer, structure, insulation, weathertight)	£35,000–£55,000
Staircase (standard, including alterations to existing floor)	£8,000–£15,000
En-suite bathroom (mid specification)	£12,000–£22,000
M&E (heating, electrical, ventilation)	£6,000–£12,000
Finishes (plasterboard, plaster, flooring, decoration)	£8,000–£15,000
Total construction	£69,000–£119,000
Professional fees (architect, structural engineer)	£8,000–£15,000
VAT (20%)	£15,000–£27,000
Total project cost (incl. VAT, fees)	£92,000–£161,000

Conclusion

A loft conversion in north London represents excellent value compared to other forms of floor area addition — it is generally achievable within permitted development rights on the rear slope, construction cost is lower per sqm than for extensions or basements, and the value added in bedroom category terms is consistently strong. For a Victorian or Edwardian house that has already been extended at ground floor level, a loft conversion completing the development of the building vertically is the next logical investment. An architect familiar with north London loft conversions will advise on the most appropriate type for the specific roof geometry, the planning constraints of the area, and the specification that achieves the best result within the available budget.