Replacing Windows and Doors: Planning, Building Regs, and What to Specify

Replacement windows and doors are generally covered by Permitted Development (PD) rights — you do not need planning permission to replace windows and doors in a residential property, provided the replacement is like-for-like in terms of appearance.

**Exceptions where planning permission IS required**:

• **Conservation Areas (CA)**:
• In a Conservation Area, replacing windows or external doors that face a public road or public open space requires planning permission (the PD right is removed by Article 4 Direction in most London conservation areas). The key test is:
• •Does the new window or door materially affect the appearance of the building?
• •If yes — planning permission required
• •Unenforced reality: Many homeowners in CAs replace windows without permission and receive no enforcement action. However, if permission was required and not obtained, this is an 'LDC issue' that will surface on a property search at sale — and a buyer's solicitor will require retrospective regularisation or indemnity insurance

**Listed Buildings**: Replacing windows or doors in a listed building requires Listed Building Consent — there are no PD rights for alterations that affect the character of a listed building. The LPA's conservation officer will want original-specification windows retained or matched in an appropriate material (often timber sashes — not uPVC).

**Flats**: Flats are more restricted under PD than houses. In a leasehold flat, the freeholder's consent is also typically required for external alterations — check the lease.

• **Conservation Area design requirements for approved window replacement**:
• Where LPA approval is required for a CA, the planning officer will typically expect:
• •Timber-framed windows that match the original style and profile (sash for Victorian, casement for earlier periods)
• •If timber is not specified, slim-profile aluminium that closely matches timber profiles may be acceptable in some cases — uPVC very rarely is in a sensitive CA
• •Matching glazing bar configuration to the original (12-pane sash, 6-over-6, 2-over-2, etc.)
• •No visible top-hinged vents (tilt-and-turn acceptable in some cases if the profile is appropriate)
• •Appropriate hardware (sash lifts, pulleys, sash horns on tipped sashes)

All replacement windows and doors in dwellings must comply with Part L1B of the Building Regulations (Conservation of Fuel and Power in Existing Dwellings). The key performance requirements are:

• **U-value requirements for replacement windows (Part L1B)**:
• •The replacement window assembly must achieve a maximum U-value of 1.4 W/m²K
• •Most modern double-glazed uPVC and aluminium window units achieve 1.2–1.4 W/m²K without specific upgrading
• •Triple-glazed units achieve 0.6–0.8 W/m²K — significantly better, with corresponding cost premium
• •Single glazing (retained in sash windows in conservation areas, for example) does not comply with Part L — but the exemption for heritage context means that in practice, retained single-glazing in listed buildings or sensitive CAs is permitted where the LPA requires original materials

• **U-value for replacement doors**:
• •Replacement external doors (solid or partially glazed) must achieve 1.4 W/m²K if more than 60% glazed, or 1.8 W/m²K if less than 60% glazed
• •Modern composite doors (GRP or aluminium-clad timber door leaf with insulated core) typically achieve 1.0–1.4 W/m²K

• **Secondary glazing as an alternative to replacement**:
• In conservation areas where the LPA requires retention of original single-glazed sash windows, secondary glazing (an inner secondary frame installed behind the existing window) is the accepted energy performance solution. Secondary glazing:
• •Does not require planning permission (it is on the inside of the building)
• •Can achieve a combined U-value of approximately 1.3–1.8 W/m²K when combined with the existing single-glazed outer window
• •Is reversible — important for listed buildings
• •Quality secondary glazing systems (Selectaglaze, Ritemp, CR Laurence) are significantly better than cheap DIY secondary glazing films

**FENSA certification**: FENSA (Fenestration Self-Assessment Scheme) is a government-authorised Competent Person Scheme for window and door installation. An installer registered with FENSA (or CERTASS, the other main scheme) can self-certify that the installation complies with Building Regulations Part L, without requiring a separate Building Control application.

• If your window installer is NOT FENSA/CERTASS registered:
• •You must apply to Building Control for a building notice before replacement works begin
• •Building Control will inspect the installation and issue a completion certificate
• •Without a FENSA certificate or Building Control completion certificate, the window replacement is uncertified — which is a problem at point of sale (solicitors check for FENSA certs for all post-2002 window replacements)

**Always use a FENSA-registered installer** (check at fensa.org.uk) or apply to Building Control if your chosen contractor is not registered.

**Frame materials**:

*uPVC*: The most commonly installed material in the UK. Low maintenance, good thermal performance, lowest cost. Profiles are bulkier than timber or slim-line aluminium. Generally not appropriate for conservation areas or listed buildings. Expected lifespan 20–30 years before yellowing or seal failure. Cost: lowest of all materials.

*Aluminium*: Slim profiles (system dependent — typical sightline 50–80mm vs uPVC 90–120mm). Good thermal performance with thermal break technology. Durable, minimal maintenance, can be powder-coated in any RAL colour. Generally more acceptable than uPVC in conservation areas. Premium over uPVC typically 40–80%. Expected lifespan 35–50+ years.

*Timber*: The traditional material for sash and casement windows. Required for listed buildings and sensitive conservation area contexts. Higher maintenance (repainting every 5–8 years). Modern engineered timber frames (laminated sections) are more stable than traditional single-piece timber. Softwood (pine/meranti) is the most economical; hardwood (iroko, oak, sapele) has higher cost and significantly longer lifespan. Cost: typically 30–60% premium over uPVC for equivalent window; higher for hardwood.

*Composite (timber-aluminium)*: Timber internally, aluminium-clad externally. The premium specification for conservation area and listed building contexts — internal appearance of timber, external durability of aluminium. Highest cost bracket.

• **Glazing specification**:
• •4-12-4 standard float glass double-glazed unit (DGU): U-value approximately 2.7 W/m²K — the old standard, no longer compliant
• •4-16-4 DGU with argon fill and low-e coating (soft coat): U-value approximately 1.0–1.2 W/m²K — the current standard for compliance
• •4-16-4-16-4 TGU (triple glazed) with argon and low-e coatings: U-value approximately 0.6–0.8 W/m²K — best thermal performance but heavier (affects sash balance weights) and higher cost
• •Acoustic glass (laminated inner pane): reduces sound transmission — specification STC 40 or Rw 40 dB typically achieves meaningful reduction vs standard DGU. Worth specifying for rooms facing a busy road in London.

• **Hardware (for sash windows)**:
• •Draught-proofing brush seals — essential for thermal and acoustic performance in sash windows
• •Sash balances: traditional weights in box frames (most appropriate for conservation contexts); spiral balances (cheaper, easier maintenance); tension spring balances (compact, used in modern mock-sash)
• •Security: BS 3621 key-lockable window locks for all ground-floor and easily accessible windows (required for Secured by Design accreditation and often for insurance purposes)

• **Approximate costs for window replacement in London (2025, supply and fit, standard uPVC unless noted)**:
• •Standard casement window (1200 × 1050mm): £600–£900 uPVC; £900–£1,400 aluminium; £900–£1,600 timber
• •Sash window (600 × 1200mm): £800–£1,200 uPVC; £1,200–£2,000 timber; £1,800–£3,000 engineered timber with draught-proofing
• •Composite front door (with frame): £1,500–£3,500
• •Aluminium sliding patio doors (2400 × 2100mm): £2,500–£5,000
• •Aluminium bi-fold doors (3600mm wide, 3 panels): £3,500–£7,000