Can I install French doors in a listed building?

Yes — French doors (especially heritage timber with multi-pane glazing bars) are often the conservation officer's preferred contemporary glazing approach for a listed building's rear addition. Listed Building Consent (LBC) is required for any glazing change on a listed building, but a sensitively detailed timber French door in a heritage profile is typically approved within 8–10 weeks. Conservation officers commonly reject aluminium, uPVC and bifolds on listed buildings; timber French doors are the safe specification.

Are French doors less secure than bifolds or sliders?

Not inherently — modern French doors (timber, aluminium or composite) achieve PAS 24 with multi-point locking, security hinges and laminated glass. The classic 'kick-in' weakness of older French doors (single mortice lock at handle height) is solved by modern multi-point locking. London insurance policies treat properly specified French doors as equivalent to other glazed door types for security risk.

Can I have a flush threshold with French doors?

Yes — flush threshold is achievable with French doors but requires careful detailing in the same way as bifolds/sliders: thermally broken low threshold profile (Reynaers Hi-Door, Schuco SI 82+ threshold), DPC and waterproofing layer beneath, drainage channel externally, falls to drain on the external paving. Cost premium for flush threshold detail: £400–£900 for a French door pair. The same detail principles apply: a 'rebated low threshold' (15–25mm upstand) is the most common compromise for French doors, offering near-flush feel with simpler weatherproofing.

Why are bifolds and sliders so much more expensive than French doors?

Three reasons. (1) Engineering complexity — bifolds have multiple hinged panels with complex hardware; sliders have heavy panels on roller carriages and large sealed glazing units; French doors have two panels on simple hinges. (2) Glass area and weight — a 4m bifold has approximately 8.5m² of double-glazed glass weighing 200kg+; French doors have 3–4m² of glass. (3) Aluminium frame manufacture — bifold and slider frames use complex extruded thermally-broken sections; French doors use simpler sections. The cost differential is largely justified by what each system delivers — bifolds and sliders open larger spans and give more dramatic indoor-outdoor flow; French doors are excellent value for narrow openings.

French Doors vs Bifolds vs Sliders London 2026 | Builderr

French doors in London — cost, spec and use cases

French doors (a pair of inward or outward opening doors, typically 1.5–2.4m wide combined) are the traditional, lowest-cost option for opening a rear elevation to a garden. London 2026 supply and install costs: timber French doors (Mumford & Wood Conservation Plus, Bereco, Westbury) 1.8m wide pair: £3,200–£5,500; 2.4m pair £4,200–£6,800. Aluminium French doors (Reynaers, AluK F82): £2,800–£4,500 for 1.8m pair. uPVC French doors: £1,800–£2,800 (not recommended for renovations above £750k property value). Glazing: standard double-glaze argon-filled low-e 28mm sealed unit; U-value 1.4–1.6 W/m²K (whole door). Use cases where French doors are the right choice: (1) Conservation area or listed building rear elevations where modern bifold/sliding aesthetic would be refused — French doors with traditional 6-pane or 8-pane subdivision in timber are the conservation-friendly default. (2) Small rear extensions (under 20m²) where a 1.8–2.4m opening is proportionate; a 4m bifold on a 20m² extension can dominate the space and look over-spec. (3) Narrow side returns: a 1.8m French door fits between structural columns where a 4m bifold cannot. (4) Budget-driven renovations: French doors at £3,500 deliver basic garden access at one-third the cost of a 4m bifold at £11,000.

Decision matrix: which door type for which extension

Use this framework for a London extension door selection. Period property rear extension in CA, opening 1.8–2.4m: timber French doors (conservation default; £3,500–£5,500). Period property rear extension in CA, opening 3–4m: timber French doors with side lights (sidelights add £1,200–£2,400) or aluminium bifold in anthracite (conservation officer permitting; £8,500–£12,000). Contemporary rear extension, opening 3m: 3-panel aluminium bifold (Reynaers CF 77; £8,500–£11,500) or French doors with side lights for value. Contemporary rear extension, opening 4–5m: 4–5 panel aluminium bifold (Reynaers CF 77 or Schuco AS FD 75; £11,000–£17,500) or 2-panel sliding (Reynaers SL38; £12,500–£20,000). Contemporary rear extension, opening 5–6m+: lift-and-slide sliding doors (Schuco ASS 70.HI or Reynaers Hi-Finity; £16,000–£30,000) — slim sightlines justify the cost on a large opening; large bifolds (6 panels+) become visually busy. Open-corner extension (90° garden access on a corner site): corner bifold (corner-jointed Reynaers CF 77; £14,000–£22,500) — dramatic open-corner effect when fully folded. Top-spec prime central London: Sky-Frame sliding doors at 21mm sightline (£18,000–£35,000 for a 4–5m opening) — Mayfair, Chelsea, Belgravia spec.

Worked examples: a £45k vs £85k vs £150k London extension

Three worked examples illustrate the door choice and its budget implications. £45,000 rear extension (15m² single-storey, basic spec, Edmonton outer London): door spec — 1.8m aluminium French doors (AluK or value-spec) £3,200 supply & install. Approximately 7% of extension cost. £85,000 rear extension (25m² single-storey kitchen-diner, Hackney): door spec option A — 3.6m aluminium bifold Reynaers CF 77, 4-panel £10,500 supply & install (12% of cost); option B — 3.6m aluminium French doors with sidelights £6,800 (8%); option C — 3m sliding door Reynaers SL38 £10,500 (12%). Option A typically chosen for family-entertaining rooms; Option C for clean visual lines. £150,000 wraparound extension (45m², Wandsworth Victorian terrace): door spec option A — 5m × 6-panel Schuco AS FD 75 bifold £16,500 (11%); option B — 5m × 2-panel Schuco ASS 70.HI sliding £19,500 (13%); option C — 5m × 2-panel Sky-Frame slim-frame sliding £29,500 (20%). Most £150k+ projects choose option A or B; Sky-Frame appears in £250k+ projects where the spec budget justifies the premium.

Mistakes to avoid when specifying doors for a London extension

Common errors that cost time, money or aesthetic value. (1) Specifying bifold or sliding before checking lintel/structural feasibility. A 5m bifold opening requires structural steel design upfront — the engineer should be consulted before door selection, not after. Late changes can require a redesigned steel and £1,500–£3,000 of rework. (2) Choosing the cheapest system without checking sightline. Visofold or AluK at 122mm sightline vs Reynaers at 99mm is visually obvious on a 4m+ bifold — the cost saving (£1,500–£2,500) is often regretted within months. Most clients prefer to upgrade after seeing both side by side. (3) Ignoring threshold detail. A standard upstand threshold (50–80mm rise) trips young children, awkwardly interrupts a wood floor running into the garden, and looks dated against contemporary expectations. Specify flush threshold detail in the structural design and the bifold supplier's installation manual. Cost: £600–£1,400. Worth it. (4) Overlooking glass area in summer. South or west-facing 4m+ bifold walls in a small extension (20–25m²) can overheat in summer — 30–40°C internal in July is reported on poorly specified glazing. Solar control glass (£200–£400/door upgrade) or external solar shading (Brise soleil, electric awning, planting) should be considered at design stage. (5) Not booking the supplier early. Reynaers and Schuco lead times can stretch to 12–16 weeks in busy periods (summer ordering for winter installation is typical). Door order should be placed once planning approval and structural design are signed off — usually 8–12 weeks before site delivery date.

French Doors vs Bifolds vs Sliding Doors for a London Extension: Which Should I Choose?