Loft Conversion Guide: Types, Costs, Planning and Building Regulations 2025

**Five types of loft conversion used in London**:

**1. Velux (rooflight) conversion**: The roof structure is kept intact and Velux or equivalent roof windows are installed in the existing slope. New structural floor joists are added to carry bedroom floor loads, rafter insulation is installed to Part L, and the space is lined and fitted. The simplest and lowest-cost type.

*Best for*: Houses with 2.2m+ head height (floor joist to ridge); where maximum volume is acceptable without extending the roof profile; where permitted development rights are intact. *Cost*: £22,000–£35,000 (single bedroom); £28,000–£45,000 (bedroom + en-suite shell) *Planning*: Almost always permitted development — no planning permission required

**2. Box dormer conversion** (rear): A full-width vertical-walled extension projecting from the rear roof slope with a flat roof. The most common London loft conversion type. Creates a full-height room across the rear 60–70% of the loft.

*Cost*: £38,000–£55,000 (bedroom + en-suite) *Planning*: Usually permitted development on the rear slope

**3. L-shaped dormer conversion**: A rear box dormer combined with a side return dormer that covers the two-storey outrigger (the step-back rear wing of a Victorian terrace). Common in London Victorian terraces.

*Creates*: Typically 35–55m² of habitable floor area — enough for two bedrooms + bathroom *Cost*: £52,000–£78,000 *Planning*: The rear dormer is usually PD; the side return may need planning if visible from a public road

**4. Hip-to-gable conversion**: Replaces the 'hipped' sloping end of a semi-detached or end-of-terrace roof with a vertical gable, dramatically increasing head height and volume at the hip end.

*Best for*: Semi-detached houses and end-of-terrace properties with four-slope roofs *Often combined with*: Rear dormer for maximum volume *Cost*: £48,000–£72,000 (hip-to-gable + rear dormer) *Planning*: Usually PD outside Conservation Areas

**5. Mansard conversion**: Replaces the entire rear roof slope with a near-vertical wall (typically 72°) and a flat roof. Creates the maximum possible loft volume. Common in inner London terraces.

*Cost*: £60,000–£95,000+ *Planning*: Almost always requires planning permission

**Checking suitability before committing**:

• •*Head height check*: Measure from ceiling joist to ridge. Minimum 2.2m for a workable Velux; 2.4m+ for a comfortable habitable room. Below 2.2m, a dormer or mansard may be needed.
• •*Roof structure*: Traditional cut roof (common pre-1970s) is straightforward. Modern trussed rafter roofs (post-1970s) need every truss modified — more complex, adds £4,000–£10,000 to cost.
• •*Water tank*: Cold water tanks in the loft must be relocated — allow £600–£1,500.
• •*Party wall*: Almost all London terraced loft conversions trigger the Party Wall etc. Act 1996. Serve notices early — the process adds 2–3 months minimum to the pre-start programme.

**When a loft conversion is permitted development (no planning permission needed)**:

Most London loft conversions are permitted development under Class B, Part 1 of the GPDO 2015. Key conditions:

• •*Volume limit*: 40m³ additional volume for terraced houses; 50m³ for semi-detached and detached. Cumulative — any previous loft alteration counts towards this limit.
• •*Front slope*: No element of the conversion may extend beyond the existing front roof slope (no front dormers).
• •*Height*: No element may be higher than the existing ridge.
• •*Side windows*: Must be obscure-glazed in any part within 1.7m of the finished floor level; non-opening within 1.7m of floor level.
• •*No balcony or veranda*.
• •*Materials*: Similar in appearance to the existing house.
• •*Not a listed building*; not on a World Heritage Site.

**When planning permission IS required**:

• •Conversion exceeds the 40/50m³ PD volume limit
• •A front dormer is proposed (visible from the highway)
• •The property is listed (also requires Listed Building Consent)
• •An Article 4 Direction removes PD rights for roof alterations — check the borough's planning portal
• •Mansard (full rear slope replacement) — almost always requires planning permission

**Certificate of Lawful Development (CLD)**:

• Even where a loft conversion is PD, obtaining a CLD from the LPA is strongly recommended if:
• •The conversion is near the 40/50m³ PD volume limit
• •Previous loft alterations may have already used some of the PD allowance
• •The property is in or near a Conservation Area
• •The property may be sold within 3–5 years (buyers' solicitors routinely request proof)

CLD fee: £206. Determination period: 8 weeks.

**Mandatory Building Regulations requirements for loft conversions**:

All loft conversions creating a habitable room must comply with the Building Regulations. Key requirements:

*Part A — Structural*: New structural floor joists (typically C24 timber at 400mm centres or LVL) to carry 1.5 kN/m² bedroom floor load. Structural engineer's calculations and Building Control drawings required.

• *Part B — Fire safety* (this is the most critical and most commonly under-specified element):
• •*Protected staircase*: The new loft staircase must be within a fire-protected enclosure (walls, ceiling, and doors with REI 30 / 30-minute fire resistance) from the loft room to the front door of the house.
• •*FD30S fire doors*: Self-closing FD30S fire doors must be fitted at the loft landing AND at the first-floor landing where the staircase passes through. These are mandatory — not optional extras.
• •*Escape window*: Where the loft room is accessed by a single stair, an escape window opening ≥ 0.33m² (minimum dimension 450mm) at a height between 800mm and 1,100mm from floor level is mandatory.
• •*Grade D LD2 interlinked fire detection*: Mains-powered interlinked smoke detectors in all habitable rooms, a smoke detector in the loft hall, and a heat detector in the kitchen.

*Part K — Staircase*: Maximum pitch 42°; going ≥ 220mm (at 270mm from narrow end on winders); minimum 2.0m headroom above pitch line; 1.9m headroom within 600mm of a wall on winder stairs serving a single room.

*Part L — Insulation*: 100mm PIR between rafters + 50mm PIR over rafters (or equivalent) to achieve a pitched roof U-value ≤ 0.18 W/m²K.

**Cost guide (London, 2025) — all figures include**: structural floor, structural engineer, roof window or dormer structure, flat roof (warm roof with PIR + single-ply membrane where applicable), Part B fire safety (FD30S doors, protected stair, escape window, Grade D LD2 detection), Part L insulation (between and over rafters to achieve ≤0.18 U-value), first-fix electrical, new staircase (dog-leg or quarter-turn winder), Building Control fees.

*Not included*: Second-fix materials (flooring, sanitaryware, tiles — client-supplied per RCB standard terms); planning fees if required; architect drawings if required; party wall surveyor fees.

| Type | Cost Range | |---|---| | Velux, 1 bedroom | £22,000–£35,000 | | Velux, 1 bed + en-suite shell | £28,000–£45,000 | | Box dormer, 1 bed + en-suite | £38,000–£55,000 | | L-shaped dormer, 2 bed + bath | £52,000–£78,000 | | Hip-to-gable + rear dormer | £48,000–£72,000 | | Mansard, 1–2 bed + bath | £60,000–£95,000+ |

• **Programme (on-site only)**:
• •Velux: 8–12 weeks
• •Box dormer: 10–14 weeks
• •L-shaped dormer: 12–18 weeks
• •Mansard: 14–20 weeks

Add 4–8 weeks for Building Regulations Full Plans approval and 2–3 months for party wall process if neighbours do not consent immediately.