How to Choose Windows for Your House Extension Project

When planning a house renovation or extension, choosing the right external doors and windows is one of the key decisions homeowners will need to make. It’s not just the look—windows and doors play a crucial role in energy efficiency, natural light, ventilation, and how the space connects with the rest of the home and garden. 

The residential window market is served by several manufacturers, each offering a wide selection of styles and models. Here are some of the most popular options:

Composite windows

Also known as alu-clad timber, they feature a fixed inner wooden frame, and an openable outer aluminium casement. The internal softwood frame can be finished in clear lacquer or painted in any RAL colour; the external frame can also be sprayed in any RAL colour, although ‘standard’ colours will be slightly cheaper. They can be hinged or sliding, with some limitations on opening sizes. These windows can be double or triple glazed, with U-values* ranging from 1.4W/m2k (the minimum allowed in the Building Regulations, can be used for most refurbishments and extensions) to  0.8W/m2k (suitable for new builds and eco-refurbishments). The main suppliers are Velfac, Rationel and Ideal Combi - they originate from Scandinavia but are now manufactured in various EU countries. 

Aluminium windows and doors

They can be hinged (patio doors), sliding or bifold. Whilst sliding door panes can be as wide as 3m each, hinged and bifold panes usually are limited to 1.2m wide. The frame thickness ranges from 90mm (entry models) to a slender 27mm for the premium casements. Their U-values typically range between 1.4 and 1.1W/m2k. These windows are mainly used for refurbishment and extensions, can be used on new builds if the glazing is not too extensive. The main suppliers in the UK are Origin and Smart. 

Crittall style windows and doors

The ‘real’ Crittall windows (still produced by the same factory) are steel frames with slimline double glazing; their thermal performance is very poor and can only be used as replacements on heritage projects. In the last ten years several manufacturers have developed thermally broken aluminium version of the Crittall windows. They are mainly hinged, but it is now possible to have the Crittall design in bifold and sliding versions. The thermal performance is typically slightly lower than that of standard aluminium windows, making them suitable for house extensions and renovations where high energy efficiency is not a priority for the client. 

Passivhaus windows and doors.

These are a more thermally efficient version of composite windows. Externally they have aluminium frames with an internal core that can be timber (which is left exposed) or Upvc clad in aluminium (giving the appearance of an aluminium window). Their U-values can be as low as 0.62W/m2k, which makes them the ideal choice for new build eco-homes and eco-renovations. Manufacturers include Internorm and Rehau. 

Timber windows and doors

The frames can be made from softwood or iroko and finished with spray paint, or from hardwoods such as oak. Most timber windows in the UK are made by local workshops, rather than modern factories, and don’t match the thermal performance of factory made casements. They are used predominantly on heritage projects (listed buildings and conservation areas) to match historic casements. These windows are usually not suitable for new build projects because of lack of thermal performance and air tightness. It is also possible to source factory-made, high-performance triple glazed timber windows and doors - but these are a niche product at the moment. 

Conclusion

Sometimes our proposals will be based around a specific type of window (because its appearance compliments the design or a certain performance is required). In other instances the design will not be specific and the clients can choose the window type they prefer, taking into account aesthetics, cost and procurement (for example, composite windows can be fiddly to order and not all builders will be familiar with their installation). 

In all projects we encourage our clients to visit in person a local showroom or a self build exhibition to see the windows in the flesh and make sure the final specification fulfils all their requirements. For aluminium windows in Bristol we work closely with Associated Windows https://www.associatedwindows.co.uk/

* The U-value measures how well a building material or element resists heat flow. A lower U-value signifies better insulation, meaning less heat is lost. 

About Us

DHVA combine award-winning contemporary design ability and attention to detail to help our clients create beautiful spaces tailored to their needs. Contact us for a no obligation discussion about your project on 0117 914 8317 or email info@dhva.co.uk.