How to Hire a Window and Door Installer

Written by James Walker · Last reviewed 15 August 2026

James Walker has 2 degrees in construction (BSc (Hons) Building Surveying · MSc Environmental Design of Buildings) and an extensive background in issue resolution, resolving over £4 billion of consumer issues.

The short answer

Replacement windows are subject to Building Regulations. You need the certification.
Use a FENSA or CERTASS registered installer who can self-certify, or budget for building control.
This trade has some of the most aggressive selling in home improvement. Never sign on the day.
Guarantees are long. Ask who underwrites them if the company ceases trading.
Conservation areas and listed buildings change what is permitted entirely.

On this page

What makes hiring a window installer different
How to handle the sales visit
What a good quote looks like
Deposits and payment stages
What usually goes wrong
Questions to ask

What makes hiring a window installer different

Two things: it is regulated, and it is sold hard.

Replacement windows are controlled fittings under the Building Regulations. They must meet Part L thermal performance, Part F ventilation (which is why trickle vents appear), safety glazing requirements and, on upper-floor bedrooms, means of escape.

Registered installers under FENSA or CERTASS can self-certify and lodge the certificate for you, usually within four to six weeks. Both are legally equivalent, FENSA is larger and better known to conveyancers, CERTASS has identical effect. If your fitter is registered with neither, you must notify building control yourself before work begins, and they inspect.

Note what the schemes do not cover: new openings, conservatories, unheated porches, conversions, new builds and extensions. Those go through building control regardless.

Certification has been required for replacement glazing since April 2002. Without it, a buyer can require retrospective regularisation, a few hundred pounds and a delay to your sale.

The selling culture is the other half. Long in-home appointments, prices that drop dramatically during the visit, discounts contingent on signing that evening, and finance offered alongside. A price that falls by a third when you hesitate was never a real price.

How to handle the sales visit

Set the terms in advance: you want a written quote and you will not be deciding on the day. Say it when booking.

If the price changes during the appointment, ask why the first figure was quoted. If a discount expires that evening, let it expire, genuine offers survive the night.

Get at least three quotes on the same specification, and insist on the specification being written down: frame material, glazing units, U-value, hardware, and what happens to the existing sills and reveals.

What a good quote looks like

Frame material and profile, glazing specification and U-value
Which windows open, and hardware specification
Trickle vents, where required
What happens to sills, reveals and internal finishes, making good is frequently excluded
Removal and disposal of old units
FENSA or CERTASS certification, and when you will receive it
Guarantee length, and who underwrites it

Deposits and payment stages

Units are made to measure, so a deposit is genuine, 25-30% is defensible. But this trade has a poor history of large up-front payments lost to company failures, so never go beyond that and never pay in full before installation.

Hold the final 10% until the certificate arrives. Like boilers and electrical work, the paperwork is part of what you bought and is what conveyancing will ask for.

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See how it works What happens if there's a dispute?

What usually goes wrong

Certification never issued, surfacing at sale
Making good excluded, leaving damaged plaster and sills
Guarantees worthless because the installing company dissolved
Trickle vents omitted where required
Deposits lost when a company failed before installation
Conservation area rules breached, requiring reinstatement

Questions to ask

Are you FENSA or CERTASS registered, and what is the number?
When will I receive the certificate?
Is making good included, and to what standard?
Who underwrites the guarantee if your company stops trading?
Is my property in a conservation area or listed, and does that change anything?
Can I have this quote in writing to consider?

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