Replacement windows: costs and choosing an installer

Hiring help for replacing windows: understand permissions, choose a properly certified installer, and get comparable quotes before signing anything.

RW-01What replacing windows actually involvesA window installer removes each old frame, fits a new one, seals and insulates around it, and makes good the plaster or render reveal inside and out. Most three-bedroom houses have 8-12 windows, so this is usually a whole-house job done over a few days rather than one window at a time. If you're also enlarging an opening, swapping to a different style, or changing a bay window's shape, that's extra structural work on top of a straight swap.

RW-02Permissions you need to check firstReplacement windows must meet Building Regulations for thermal performance, safety glass, and ventilation. Most installers are registered with a competent person scheme (FENSA or CERTASS), which lets them self-certify the work instead of you applying to Building Control separately. If your installer isn't registered, you must notify Building Control yourself before work starts, which costs more and takes longer. Listed buildings need listed building consent for any window change, and some conservation areas restrict materials like UPVC — check with your council's planning department before you commit to a style.

RULES

KNOW THE RULES

Window replacement is covered by Building Regulations (thermal, safety, ventilation). Use a FENSA- or CERTASS-registered installer so the work is self-certified, or notify Building Control yourself. Listed buildings need listed building consent. Conservation areas may restrict materials. HomeGraft has a 'Know the rules' guide on planning permission and listed buildings.

RW-03Who to involve, and in what orderFor a standard like-for-like swap, you only need a window installer — get quotes from two or three FENSA- or CERTASS-registered companies. If you're enlarging an opening, removing a load-bearing section of wall, or changing a bay window's structure, bring in a structural engineer first to design the opening and specify a lintel; the installer or builder then works to that design. Never let a builder enlarge an opening without engineer sign-off — it's a common way structural problems get missed.

RW-04Realistic costs and timescalesFor an average three-bedroom house outside London, expect roughly £4,000-£9,000 for UPVC windows throughout, £7,000-£14,000 for aluminium, and £8,000-£15,000+ for timber. A single replacement window typically costs £400-£1,200 depending on size and material. London and the South East usually run higher. Costs climb towards the top of the range for timber or aluminium frames, bay windows, triple glazing, bespoke sizes, scaffolding access, or any structural opening work. Lead time from survey to fitting is typically 2-6 weeks for standard UPVC and 8-12 weeks for bespoke timber; fitting the whole house usually takes 1-3 days.

RW-05What a good quote should containA proper quote lists the number, size and style of each window, the frame material and glazing spec (U-value), who removes and disposes of the old frames, and who makes good the reveals inside and out. It should state the competent person scheme registration number, confirm Building Regulations compliance certificate will be issued on completion, and give a fixed price, not just a deposit figure and 'balance on completion'. Ask for the FENSA or CERTASS certificate reference before you pay the final invoice, not after.

RW-06Questions to ask, and red flagsAsk: which competent person scheme are you registered with, can I see your certification, what's your guarantee on the frames and the installation separately, and who's liable if the reveal plaster cracks later? Ask for two recent local references and, ideally, go and look at finished work. Red flags: cash-only deals, no scheme registration, pressure to sign on the day for a 'discount', vague written quotes, and no fixed completion date.

RW-07What commonly goes wrongThe most common problems are poor sealing around frames causing draughts or damp later, damaged plaster reveals not made good properly, and installers who aren't scheme-registered leaving you to sort Building Regulations compliance yourself after the fact. Bay windows and bespoke sizes sometimes arrive wrong or take far longer than quoted. Always get the completion certificate in writing before paying the final balance.

RW-08If you live in Wales, Scotland or Northern IrelandWales has its own Building Regulations (Part L Wales), and planning rules can differ slightly for conservation areas — check with your local authority. In Scotland, a straight replacement window usually doesn't need a building warrant, but making a new opening or changing its size does, so check with your council's building standards team. Northern Ireland has its own Building Regulations, so ask your council's building control whether your installer's certification covers work there.

RULES

KNOW THE RULES

In Scotland, a building warrant is needed for new or enlarged window openings, but usually not for a straight replacement. Wales and Northern Ireland have their own Building Regulations. Always confirm the specific requirement with your local authority before work starts.

KNOW THE RULES

Before you start: Do I need Building Regulations approval to replace windows or doors? · What needs listed building consent?

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