Fitting new windows

Fitting new windows is a great way to update your property's façade, and, depending on he age of your home, it doesn't have to be too complicated. Michael Holmes explains how to install new windows, the costs and what regulations you need to consider.

Fitting new windows is a great way to update your home’s exterior. Renovation expert Michael Holmes explains how.

Removing old windows and fitting new ones isn’t a difficult DIY task, especially if they’ve been made to measure. However, period timber windows should only be replaced if they’re beyond repair. Modern softwood windows that have rotted are worth replacing, as are failed or broken uPVC ones, which usually can’t be repaired.

Fitting windows is a two-person job, or more for larger windows, and it will take 3-4 hours to fit one. If the windows are above ground floor level, you’ll need a scaffold tower, which can be hired for around £25 per day – visit Always consider safety for a job like this and wear a hard hat, goggles, workboots and gloves.

Budget £200-£500 for each pre-glazed off-the-shelf softwood window, with a similar price for uPVC. A bespoke sash window will cost £1,000-£1,400.

Preparation is key

  • Remove the old window first by removing any frame fixings and preparing the opening, usually brick or block, but possibly timber.
  • If the window is fitted from the inside, you’ll need to remove any internal sills and window boards and replace these later.
  • If you’ve bought off-the-shelf timber windows that are the nearest available size to your window openings, you’ll need to make some small alterations. You could add or remove a course of bricks at sill level, or pack out the opening at the head and sides with a treated softwood subframe, which can be painted in later.
  • If your property has cavity walls, insulated cavity closers should be fitted around the opening to seal it. Cavalok makes a retrofit cavity closer.

Fitting the windows

  • Fit the frame at the same depth in the wall as the previous windows – use packers (timber wedges) to hold the frame in place and get it level, and tight up to the window head. You can then side fix the frame into the walls using 100mm frame fixings or metal strapping screwed to both the frame and the walls.
  • Handi-Foam (expandable urethane foam) is often used around frames to fit them in place and seal them at the same time – any excess can be cut away easily.
  • If the frame isn’t a perfect fit, apply external mouldings to cover any gaps./Seal around the frame using an appropriately coloured mastic sealant./Refit internal window sills or boards and make good any damaged plasterwork.
  • If the frames are not pre-glazed, fit the double glazed units and apply the glazing bars according to the manufacturer’s instructions (internally glazed windows are more secure).
  • If the windows are timber and not pre-finished, decorate the frames using micro-porous paint or varnish.

Rules and guidelines

Planning permission isn’t normally required to replace a window on a like-for-like basis, even on a listed property. If you’re changing to double glazing, or changing the material – for example, timber to uPVC – permission will be needed on a listed building and any property where permitted development rights (PD) have been restricted or removed, such as some conservation areas and other designated areas). Flats don’t have PD rights.

New windows must comply with Part L of Building Regulations – Energy Efficiency, which means in most cases the window must be double glazed. Exceptions can be made for listed buildings and period houses. If you do it yourself, there’s a fee payable to the local authority – £100-£200 inc. VAT – to check the work. The fee isn’t payable if you use a FENSA registered installer who can certify their own work, which is why it can be cheaper to use a professional if you are installing only one or two windows.

Michael HolmesMichael Holmes’ expert advice

  • Measure the window opening inside and out at the top, middle and bottom. Your actual window size will be around 1cm smaller than the height and width of the opening (5mm play all round).
  • Check the opening is square by measuring from corner to corner – the dimension should be identical. If out of square, measure to fit the smallest width and height and pack out the frame later.
  • Most windows are fitted from outside, but some (especially sash windows) sit behind the external walls and the opening size is probably larger inside than out.

In the gallery: All of the uPVC windows were replaced in this detached property with aluminium-framed alternatives and full-height square bays.