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Replacement Windows: Everything You Need to Know

Timber frame casement replacement windows
(Image credit: Westbury Windows & Joinery)

While repairing is always preferable when renovating a house, there are times when replacement windows are the only option — perhaps the originals are beyond saving or poorly-matched windows have been installed in their place by previous owners.

But windows are key in setting the overall tone for your home’s appearance, harmonising with roofing materials and other exterior finishes to establish its architectural character, and so it is important that if you are specifying replacement windows, you choose wisely. 

Here, we explore the key issues to think about.

Do I Need Planning Permission for Replacement Windows?

Permitted Development (PD) rules, so you won’t have to apply for planning consent. “What you can install depends on whether the property is listed, in a conservation area or subject to any other local planning conditions,” says Steve Brattle, divisional sales director at Anglian Home Improvements.

Some local authorities are more flexible than others, however, so it’s worth checking with your local council regarding the style, frame material and opening method of your new units before going full steam ahead.

If you’re considering switching to a different style, do your homework first. “To retain or add value to your home, it’s worth honouring the integrity of materials and methods that would have been used in the era when it was built,” says Chris Herrington, technical manager at Ventrolla.

Timber alternative windows from Thames Valley Window Company

This thatched cottage features Evolution Storm 2 Timber Alternative windows (priced from £500 per window), supplied and fitted by Thames Valley Window Company. The finish of the PVCu frames mimics the appearance of natural wood. (Image credit: Thames Valley Window Company)

PD entitlements do allow you to alter, enlarge and even introduce new windows to a building. Again, it’s important to check with your local council how far you can go with these sorts of alterations before you need to apply for planning permission. If you are going down this route, it’s also wise to consult with an expert who can advise on a window size and style that suits your property.

Building Regulations for Replacement Windows

Replacement windows fall under the scope of the Building Regulations, and must adhere to standards laid out for thermal performance, security, ventilation and means of escape. Approved Document L-1B deals with issues surrounding energy efficiency and heat loss, stipulating that replacement window units (including the frame) must achieve whole-unit U values of 1.6W/m2K or less.

Approved Document Q covers windows’ security requirements, ensuring units are sufficiently secure to withstand a manual attack. For peace of mind, look out for products that come with

Secured By Design accreditation. “The key certification to look out for is FENSA, which stands for Fenestration Self Assessment Scheme,” says Andy Young, founder and director at Create Bespoke. “It’s important for installers to be FENSA registered, as it means they can secure Building Regs approval through self-certification. It will also mean work is fully compliant and registered with the local council, which is an important step if you come to sell the property.”

Choosing the Right Material for Your Replacement Windows 

Steel replacement windows

These Bespoke Clement EB24 steel windows have been designed to match the originals. The entire window pictured here cost £3,600 to supply and install. (Image credit: Clement Windows)

“The typical choices for window frame materials in traditional projects are timber, aluminium and heritage Crittall [steel frame] style,” says Ryan Schofield, managing director at Thames Valley Window Company. “With increased colour and finish options now available, PVCu is also a realistic alternative.” 

Timber Replacement Windows

Timber is the natural choice for many schemes, and is particularly apt if you’re keen to create an authentic look when renovating a heritage property. As well as possessing innate character and warmth, this material is a natural insulator that will help you achieve better whole-unit U values.

Softwood and hardwood versions are available, with the latter falling into a higher price bracket. 

“When using modern timber products, a factory finish is typically better than finishing by hand,” says Ryan Schofield. “It should mean the window has been constructed in optimum conditions, where humidity levels are perfect and the temperature is regulated so the timber remains stable. This means the units are less likely to suffer from any warping or twisting due to drying out.” 

If you’re opting for a painted finish, make sure a microporous paint is used, as this will allow the timber to breathe and will prevent the paint from lifting and splitting.

In some cases, installing primed units and then painting the window frames once they’re in situ may be the better option. If in doubt, speak to your chosen installer to see which option is the best for your project.

Timber frame casement replacement windows

Westbury Windows & Joinery supplied the new casement windows for the renovation of this Arts and Crafts-style house. The Accoya frames have been finished in three spray-applied coats of Westbury white, using a Teknoc water-based microporous paint. (Image credit: Westbury Windows & Joinery)


In terms of maintenance, PVCu requires the least upkeep, as repainting will not be required. However, every six months or so it’s worth cleaning any debris from the interior and exterior of the unit, as well as wiping all surfaces clean — this goes for timber units, too.

If you have installed hand-finished timber windows, these will need to be repainted or varnished every five years or so to ensure a weathertight seal. 

Factory applied finishes, on the other hand, should last a minimum of five years. “However, over time a fresh application will be required to maintain a ‘new’ appearance,” says Ryan Schofield.

Aluminium Replacement Windows

Aluminium is an ideal choice for large window openings as it’s lightweight, but strong. “It offers slim, contemporary looking frames and is low maintenance. It’s available in many colours and is known as the ‘green’ metal because it’s 100% recyclable,” says Matt Higgs, director and co-owner of Kloeber. “It’s not as thermally efficient as some timber types, depending on the size/type of the thermal break.” 

As a framing material, aluminium is generally more expensive than plastic and some softwoods. 

Alu-clad Options

Alu-clad surrounds, featuring an engineered timber interior in softwood or oak, with an exterior finish of aluminium, are another option. 

“Aluminium offers a modern-looking exterior with a warm timber interior. You get all the benefits of timber and aluminium in one product,” says Matt Higgs. “This solution comes with great insulating values, with the warmth of timber on the inside and the low-maintenance of aluminium on the outside. The windows are very strong and secure, too, but may not suit all period properties.”

Finding a Supplier for Your Replacement Windows

Man replacing a window

The joiners and carpenters at Ventrolla are skilled craftspeople, experienced in working with wood. The company installs fully primed units to site to be painted and finished in situ for an authentic look. (Image credit: Ventrolla)

Installing new windows as part of your renovation represents a significant investment, so tracking down a reputable supplier is vital. Don’t rush this phase of your project, as diligent research now will pay dividends later — especially with such a wealth of local joiners and national companies to choose from.

“Recommendation is always a great way to find the right company,” says Peter Clement, chief executive at Clement Windows. “You should also check if the company is a member of the Glass and Glazing Federation (GFF). The GGF have schemes in place to safeguard your deposit. Plus, their members must work to a strict Code of Practice, including adherence to nationally recognised performance criteria, so you can be sure you’re using a reputable firm.”

Going for a supply and fit service makes good sense. That way, if there are any issues when the new windows are delivered, the fault lies with the provider and not you.

While it’s perfectly acceptable to take rough measurements for the new windows yourself, it’s best to leave the final measuring up process to the professionals. 

PVCu replacement windows from Anglian Home Improvements

Featuring low-maintenance PVCu frames, Anglian Home Improvements’ White Knight Stormproof Casement Window is available with both double and triple glazing. (Image credit: Anglian Home Improvements)

Before work begins, it’s also worth reading the small print to double check exactly what’s included in the agreement with your chosen supplier/installer. “Items like ‘making good’ around the perimeter of the frame, both internally and externally, need to be discussed directly between you and the window contractor,” says Peter from Clement Windows.

“This process will differ from house to house, so you need to understand exactly what the contractor has allowed for.”

Product Guarantees

Before settling on one company it’s wise to set aside plenty of time to investigate product guarantees that are included as part of your purchase. This is an element that varies drastically between suppliers and systems. 

“If you’re looking at timber windows, for example, the guarantee can depend on the type of timber being used,” says Andy from Create Bespoke. 

“Most industry suppliers use sapele at the lower end or another similar hardwood, which will usually come with a 10-15 year guarantee. This can jump to 25 years when suppliers use a modified timber like Accoya, which is becoming increasingly popular due to its rot-resistant qualities.”

How Much Do Replacement Windows Cost?

The price of new windows varies significantly, depending on the supplier, style and frame material. “Obviously, the bigger the window, the more bespoke the customer’s requirements and the more intricate the work required, the more the project would cost,” says Chris Herrington.

For example, for a renovation, draughtproofing and new double-glazed sashes in a Ventrolla unit measuring approximately 1.5 x 1m in size, prices start from £1,000 on a supply and fit basis. For a new sash window with a box frame, again on a supply and fit basis, Ventrolla’s prices start from £1,500 per unit.

When budgeting for a whole house’s worth of new windows, the cost can vary dramatically.

Casement traditional style windows

(Image credit: Thames Valley Windows)

“It’s hard to estimate a cost, as so much depends on the design of the windows, the sizes and the number of units,” says Robert from Westbury Windows & Joinery. 

“For a typical four-bedroom house with new windows throughout, for our products, homeowners would be looking at a minimum of £15,000, up to around £30,000.” 

If you’re working to a tight budget, and conservation guidelines allow, installing high-quality PVCu units instead of timber products could be one way to save money. “Another option is to choose lower cost windows at the rear of the property. For instance, having timber designs at the front and PVCu at the back,” says Ryan from Thames Valley Windows. 

Should I Make Existing Openings Bigger When Replacing Windows? 

Increasing natural light in a period property is frequently high on the list of why homeowners look to enlarge a window opening. However, bear in mind the passing of light in modern glazing is far superior to that of the past, so just a simple swap could suffice. 

(MORE: 15 Genius Ideas for Bringing the Outside In)

Although a significant structural alteration, enlarging a window shouldn’t be too complicated. The masonry above must be supported while the walls are cut out and a suitable new lintel installed above the enlarged opening. 

Top floor openings should be relatively light (likely supported by a timber wall plate) but old lintels further down the wall may be supporting very substantial loadings. 

Flush window replacements on period house

(Image credit: Evolution Windows)

However, high performance windows may be heavier and the wall must be able to support this. If only changing the sill level, it’s unlikely to have an impact but triple-glazed units might be unsuitable for some refurbishment projects. 

Conversely, creating smaller openings is largely a cosmetic exercise as the existing lintels or masonry will be disturbed. Fitting cavity closers (lightweight and rigid PVCu profiles) to the reveals of the new opening is advised so cold can’t easily ‘bridge across’. 

Repairing Walls After Installing Replacement Windows

New windows in period house

These bespoke EB24 diamond leaded light steel windows by Clement Windows cost £3,000 on a supply and fit basis (Image credit: Clement Windows)

Taking out and replacing old windows will cause damage to the surrounding walls so you’ll need to make good, internal and externally, once the new windows are in. 

Brick/Masonry Walls

With brick walls, try to widen the opening to a full (or half) brick dimension so half of the surrounding bricks are left uncut, while the other half can be removed, cut, and relaid for a neat finish. 

Rendered Walls

If the edge of a rendered wall is cut neatly there will be minimal patching. Installing a corner bead to protect the exposed corner is essential but a render patch could be restricted to a narrow strop at the edge of a window until the render is replaced or repainted in the future. 

Timber-clad Walls

Finish each new reveal of a timber clad elevation as simply as possible, usually with a single vertical board (as long as ventilation paths are maintained). Avoid horizontal cladding around reveals with mitred corners as they will distort quite quickly. 

Rebecca is an experienced homes and interiors writer with a passion for Victorian architecture. Her dream is to extend an 1800s house