Double Glazing Kent: the 1.4 U-Value Every Quote Must Hit

Getting double glazing in Kent quoted is easy. Getting three quotes you can actually compare is not, because most of what separates a good installation from a bad one never appears on the quotation: the U-value of the finished window, who signs off the building regulations, whether trickle vents are included, and what happens to your deposit if the firm folds. This page covers what a Kent quote has to satisfy by law, how to check the company before you check the price, and the questions that make two “similar” prices stop looking similar.

Table of Contents

The standard every Kent quote has to meet

Building regulations have applied to all replacement glazing since 1 April 2002. In England, a replacement window in an existing home has to reach 1.4 W/m2K, or Window Energy Rating Band B, under Approved Document L Volume 1. The number is small and it is the single most useful thing to ask for in writing, because everything else on a quote is marketing.

Panel summarising the building regulations that apply to replacement windows in England, including the 1.4 W/m2K standard — The building regulations a replacement window has to satisfy in England. Panel by Contemporary Structures. | Panel by Contemporary Structures

Two details are worth knowing because installers still get them wrong. There used to be a transitional concession letting timber windows meet a softer 1.6 W/m2K or Band C. It expired on 14 June 2023. If a Kent joinery firm quotes you timber at 1.6, they are working from an arrangement that ended three years ago. And where the character of the building genuinely prevents meeting the standard, Approved Document L gives a heritage route instead: a centre-pane U-value no worse than 1.2 W/m2K, or single glazing supplemented with low-emissivity secondary glazing. Very few installers volunteer that, and in a Kent conservation area it can be the difference between a refusal and a consent.

The 2026 edition of Approved Document L takes effect on 24 March 2027 for work not connected with higher-risk buildings, and it keeps the same 1.4 W/m2K or Band B figure for replacement windows. So the standard you are buying to today is not about to be superseded.

Trickle vents, and why the cheap quote sometimes leaves them off

Approved Document F Volume 1 has applied since 15 June 2022, and it is the reason your new windows will have vents along the head even if your old ones did not. Where the original window had trickle vents, the replacement’s vents must be no smaller and must be controllable. Where it did not, the guidance sets minimum equivalent areas: 8000mm2 in habitable rooms and kitchens, 4000mm2 in bathrooms, dropping to 4000mm2 in habitable rooms where the home has continuous mechanical extract ventilation. Historic and listed dwellings, and buildings of vapour-permeable construction, are expected to comply where reasonably practicable rather than absolutely, which is a conversation to have with building control early rather than late.

If one quote is noticeably cheaper, check whether vents are on it. Adding them afterwards means going back to the frames.

The certificate you should be asking about

There is no such thing as a FENSA certificate, and the trade knowing this matters more than it sounds. The legal document is a Building Regulations Compliance Certificate. Any government-authorised competent person scheme can issue one: FENSA, Certass and the others all operate under the same licensing framework. An installer who is not registered with a scheme has to put the job through a building control application with your district council instead, which is legitimate but costs more and takes longer.

Two practical points. The certificate should reach you within 8 weeks of completion, and it shows up in solicitors’ searches when you sell, which is when missing paperwork turns into a real problem. Competent person schemes also carry insurance-backed warranties and a complaints procedure, which is worth more than a company’s own “20 year guarantee” if that company stops trading.

When a window swap in Kent needs planning permission too

Replacing windows on a house is usually permitted development, so building regulations are the only hurdle. It stops being that simple in three situations, all common across Kent:

Listed buildings. Listed building consent is separate from planning permission and is needed for the work regardless of what it looks like from the street.
Article 4 directions. A council can withdraw permitted development rights for an area or a property, and window replacement is one of the first things such directions cover. Tunbridge Wells Borough Council, for example, has made Article 4(2) directions affecting properties in the St James area.
Conservation areas and flats. Designated areas bring extra restrictions, and if you are replacing the windows of a flat rather than a house, permitted development rights do not apply in the same way at all.

Kent has twelve district and borough councils, and Medway is a separate unitary authority, so the local planning authority you need is the district one: Ashford, Canterbury, Dartford, Dover, Folkestone and Hythe, Gravesham, Maidstone, Sevenoaks, Swale, Thanet, Tonbridge and Malling, or Tunbridge Wells. One phone call to their planning team before you sign is free. A retrospective enforcement notice is not.

Checking the firm before you check the price

Three checks, none of which takes long.

Trading Standards Checked. This is Kent County Council’s own safe trader scheme for the home improvement sector, and the only one vetted by Kent Trading Standards. Membership involves credit and insolvency checks, verification of the registered trading address, public liability insurance, mandatory accreditations, and ongoing monitoring of complaints made to the Citizens Advice consumer service. Searching it is the fastest way to filter a list of Kent installers.

The competent person register. Search the installer on the scheme they claim to belong to. If they are not on it, they cannot self-certify your job, whatever the van says.

Companies House. Look at how long the limited company has existed and whether there is a trail of dissolved predecessors at the same address. A “30 year guarantee” from a company incorporated last year is a piece of paper.

Your cancellation rights, and the carve-out nobody mentions

If you sign in your own home, that is an off-premises contract under the Consumer Contracts (Information, Cancellation and Additional Charges) Regulations 2013, and the normal cancellation period runs for 14 days. Here is the part that catches people: regulation 28(1)(b) disapplies that Part of the regulations to the supply of goods made to the consumer’s specifications or clearly personalised. Made-to-measure window frames sit squarely in that carve-out.

So the 14 days you assume you have may not exist for the windows themselves. Do not rely on it. Get the cancellation and deposit terms written into the order, ask what happens if the survey finds something the salesperson did not, and never pay a large deposit in cash to someone who called at the door.

What actually moves the price of a Kent window job

Anyone quoting a per-window figure over the phone has not seen your house. The variables that genuinely change the number are worth knowing so you can tell a fair quote from a padded one.

Frame material. uPVC, aluminium and timber sit at very different price points for the same opening, and they age differently.
Glass specification. Low-e coating, argon fill, warm-edge spacer bars and the choice between double and triple glazing all move both the price and the U-value.
Safety and escape glazing. Critical locations need toughened or laminated glass, and first-floor bedrooms may need an escape opening of a set size. Both are regulation, not upsell.
Access. A three-storey Victorian terrace with no side access is a different job from a bungalow.
Making good. Ask specifically whether internal plaster reveals, external silicone and rubbish removal are included, because that is where quotes quietly diverge.
Heritage detailing. Astragal bars, slim sightlines and sash horns cost real money, and in a conservation area you may not have a choice.

Ask each firm to price the same written specification rather than their own. If you want that specification drawn up properly, or a second opinion on a quote you have already been given, Contemporary Structures works across Kent on windows, doors and glazed extensions.

Frequently asked questions

Do I need a FENSA certificate for double glazing in Kent?

You need a Building Regulations Compliance Certificate. FENSA is one of several government-authorised competent person schemes that can issue one, and Certass and others issue exactly the same document. What matters is that your installer belongs to some authorised scheme, or that the work goes through your district council’s building control instead.

What U-value should my new windows be?

1.4 W/m2K, or Window Energy Rating Band B, is the minimum for a replacement window in an existing home in England. Better is available and worth asking about, but anything worse is not compliant unless the heritage route applies to your building.

Do I need planning permission to replace windows in a Kent conservation area?

Often not, but you have to check, because conservation area designation brings extra restrictions and many Kent councils have Article 4 directions that remove permitted development rights for exactly this work. If the building is listed, you need listed building consent whatever the appearance. Ask your district council’s planning team before you order.

Can I refuse trickle vents in my new windows?

Not usually. Approved Document F has required background ventilation in most replacement windows since 15 June 2022, and an installer who leaves them off to win the job is putting your compliance certificate at risk. Historic and listed dwellings are treated with more flexibility, which is a matter for building control, not the salesperson.

How long should the whole job take?

Survey to installation is usually several weeks because the frames are made to your measurements, and the fitting itself is often one or two days for a typical house. The certificate is the last piece, and it should arrive within eight weeks of completion. Chase it if it does not.

Double Glazing Kent: the 1.4 U-Value Every Quote Must Hit

The standard every Kent quote has to meet

Trickle vents, and why the cheap quote sometimes leaves them off

The certificate you should be asking about

When a window swap in Kent needs planning permission too

Checking the firm before you check the price

Your cancellation rights, and the carve-out nobody mentions

What actually moves the price of a Kent window job

Frequently asked questions

Do I need a FENSA certificate for double glazing in Kent?

What U-value should my new windows be?

Do I need planning permission to replace windows in a Kent conservation area?

Can I refuse trickle vents in my new windows?

How long should the whole job take?

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