Question 1

How much does a loft conversion cost in London in 2026?

Accepted Answer

In 2026 a standard rear dormer loft conversion in Greater London costs £55,000-£90,000. A mansard conversion (full-width rear with vertical walls) costs £80,000-£110,000. An L-shaped dormer on a Victorian terrace typically costs £75,000-£105,000. A Velux (rooflight-only) conversion is the cheapest option at £35,000-£55,000 but provides less usable headroom. Hip-to-gable with rear dormer on a semi-detached costs £65,000-£95,000. All prices include structural steels, staircase, insulation, electrics, plastering and decoration but exclude bathroom fit-out (second-fix items are client-supplied).

Question 2

What types of loft conversion are there?

Accepted Answer

The main types are: (1) Velux/rooflight - skylights installed into the existing roof slope, cheapest but limited headroom; (2) Rear dormer - a flat-roofed box extension at the rear, the most common type in London; (3) L-shaped dormer - extends across the rear and wraps around the side on Victorian terraces; (4) Hip-to-gable - converts the sloped side roof to a vertical gable wall, common on 1930s semis; (5) Mansard - replaces the entire rear roof slope with near-vertical walls, maximum space but usually needs planning permission; (6) Modular/prefabricated - factory-built loft pods craned into position, faster but more expensive.

Question 3

Do I need planning permission for a loft conversion?

Accepted Answer

Most rear dormer conversions fall under Permitted Development (PD) provided: the volume added does not exceed 40m3 (terraced) or 50m3 (detached/semi), the dormer does not extend above the existing ridge, materials match the existing house, no balconies or raised platforms, and side-facing windows are obscure-glazed and non-opening below 1.7m. Mansard conversions almost always require full planning. Conservation areas, listed buildings and flats always need planning permission. We recommend obtaining a Lawful Development Certificate (LDC) from the council even for PD work - it costs around £103 and provides legal certainty for future sales.

Question 4

How long does a loft conversion take from start to finish?

Accepted Answer

The total programme is typically 16-22 weeks. Pre-construction (architectural drawings, structural engineering, Building Control application, party wall notices if needed) takes 4-8 weeks. A standard rear dormer takes 8-10 weeks on site. A mansard or L-shaped dormer takes 10-14 weeks. The first 2-3 weeks involve scaffolding, steel delivery and the structural opening; you can live in the house throughout but expect noise and dust during the first-fix phase.

Question 5

Can I live in my house during a loft conversion?

Accepted Answer

Yes - loft conversions are one of the least disruptive major building projects. Work is largely contained above ceiling level. The main disruption comes during the staircase installation (typically 2-3 days when the landing area is affected) and the first-fix electrical work when circuits are run from the consumer unit. Expect some dust and noise, particularly during the first 3-4 weeks when the roof is opened, steels are installed and the dormer structure is built. RCB erects dust sheets and temporary partitions to minimise disruption.

Question 6

What building regulations apply to loft conversions?

Accepted Answer

Key Building Regulations for loft conversions include: Part A (Structure) - steel beams and floor joists must be calculated by a structural engineer; Part B (Fire Safety) - the new staircase must be enclosed in 30-minute fire-rated construction, with fire doors to all habitable rooms opening onto the new stair and interconnected mains-wired smoke alarms; Part L (Energy) - minimum 270mm mineral wool or equivalent insulation in the roof; Part K (Stairs) - minimum headroom of 2m over the centreline, max pitch 42 degrees, min width 620mm for a fixed stair; Part P (Electrics) - all new circuits must be tested and certified by a registered electrician or inspected by Building Control.

Question 7

Do I need a party wall agreement for a loft conversion?

Accepted Answer

You need a Party Wall Notice if the loft conversion involves cutting into or building on a shared (party) wall - which most terraced and semi-detached loft conversions do, because the new steel beams typically bear on the party wall. You must serve notice at least 2 months before starting work. If your neighbour consents in writing within 14 days, no surveyor is needed. If they dissent or do not respond, each side appoints a party wall surveyor (or agree on a single Agreed Surveyor). Costs are typically £900-£2,000 per neighbour and are paid by the building owner.

Question 8

Will a loft conversion add value to my house?

Accepted Answer

A well-executed loft conversion in London typically adds 15-25% to a property's value, significantly more than the cost of the works. An additional bedroom with en-suite on a 3-bed terraced house in South East London can add £60,000-£120,000 to the sale price (based on 2025-2026 comparable sales). The return is strongest when converting from 2-bed to 3-bed or 3-bed to 4-bed, as these are step-change improvements in the property bracket. Properties with head height challenges or no en-suite see lower returns.

Question 9

What is the minimum head height for a loft conversion?

Accepted Answer

Building Regulations require a minimum headroom of 2.0 metres (2m) measured over the centreline of the new staircase. For the room itself there is no regulatory minimum, but practically you need at least 2.2m at the ridge to create a usable room after insulation and floor build-up. We measure existing ridge height during the initial site survey - if the ridge is below 2.4m, a loft conversion is rarely viable unless the roof structure is raised (which adds significant cost and usually requires planning permission).

Question 10

What staircase options are available for a loft conversion?

Accepted Answer

The staircase must comply with Part K of the Building Regulations. Standard options include: (1) A straight flight from the existing landing (requires the most space but is cheapest); (2) An L-shaped or winder stair that turns 90 degrees (the most common solution, typically positioned over the existing staircase); (3) A spiral staircase (only permitted if it serves a single room and not the only staircase to a habitable room). The staircase typically costs £2,500-£5,000 for softwood or £5,000-£10,000 for hardwood/oak. RCB designs the staircase position to minimise the impact on existing rooms - in most terraced houses it rises from the rear bedroom or half-landing.

Question 11

What insulation is required in a loft conversion?

Accepted Answer

Building Regulations (Part L, 2021 edition) require a U-value of 0.18 W/m2K for the roof slope, which equates to approximately 120mm of rigid PIR insulation board (like Celotex/Kingspan) between and under the rafters, or 270mm of mineral wool between and over ceiling joists. Most contractors use a combination: rigid board between rafters with a vapour control layer and plasterboard over. The floor must achieve a U-value of 0.22 W/m2K if it separates heated from unheated space. Walls in dormer cheeks typically need 100mm rigid insulation. All insulation must be continuous with no cold bridges at junctions.

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