Casement window replacement is the commonest glazing job in British housing and it is more involved than it looks from the pavement. The window that comes out is bedded into a wall that was built around it, and the window that goes in has to meet current requirements for thermal performance, for ventilation and for safety glazing that the original probably did not. Knowing the sequence lets you judge whether a quote has priced the whole job or only the visible part of it.
The survey decides the job, not the sales visit
A sales visit produces a price and a technical survey produces a specification. The surveyor measures every opening properly, checks what the frame is fixed into, looks at the internal finish that will be disturbed, and works out how the fitters will reach anything above the ground floor. It is normal for the survey to change details and occasionally the figure, and a firm that never surveys before manufacture is taking a risk that ends up being yours.
Ventilation has to be maintained, which usually means trickle vents
When windows are replaced, the ventilation the room had must not be reduced, and in practice that generally means background ventilators built into the new frames. Homeowners are often surprised by this because their old windows leaked enough air to ventilate the room without anybody planning it. Vents are visible on the frame and they can be specified in different positions, so it is better to discuss them at survey than to discover them on delivery.
Some panes have to be safety glass
Glazing in critical locations, which broadly means low down where somebody could fall against it, and in and beside doors, has to be toughened or laminated rather than ordinary annealed glass. This is a requirement rather than an upgrade, and it applies whether or not the window it replaces complied. If a quote lists safety glass as an optional extra on a full height window, the firm has either misunderstood the job or is hoping you will not ask.
Removal and making good is where quotes diverge
Getting an old casement out disturbs the plaster at the reveal, frequently damages or reveals a tired internal cill, and always leaves external sealing to do. Where the old frame was timber bedded into a solid wall, more comes away than anybody wants. Whether the installer makes this good, to what standard, and whether decoration is included, is the largest practical difference between two quotes for what appears to be the same set of windows.
Registration, certificate and guarantee
Replacement windows are notifiable building work in England and Wales. An installer registered with a competent person scheme certifies its own work and registers it, and you receive a certificate. Keep it with the property paperwork, because it is asked for when the house is sold and it is far easier to file now than to reconstruct later. Ask separately what the workmanship guarantee covers and who stands behind it if the company stops trading.
The advantages and disadvantages of flush casement windows
A flush casement closes level with its frame, as a timber window does, instead of standing proud of it. On a period or traditionally detailed house that single difference is the one most people notice, and it is why the style has taken over the upper end of the replacement market. The costs are real: tighter manufacturing tolerances and a higher price than a stormproof casement, a marginally smaller clear opening for the same hole, which matters where the window is an escape window, and on some systems a slightly less forgiving seal line in driving rain. On a modern estate house the visual gain is small enough that the money is usually better spent on the glass specification.
Questions people ask about casement window replacement
Can I keep my existing frames and change only the sashes?
Sometimes, if the frames are sound and the system is still made. It is a smaller job and a smaller price, but it inherits whatever condition the frames are in, so it is worth a straight opinion from the surveyor rather than a preference from you.
How long does a house of casements take to fit?
It depends on the number of openings and the access rather than on the size of the house. What matters more for planning is that openings stand open while frames are changed, so it is worth asking how many will be open at once and in what order.
Will the new windows look the same as the old ones?
Almost never exactly. Modern profiles are deeper and the frame you see is usually wider than a slim timber original. If matching matters, ask to see the actual profile rather than a photograph, and ask about flush casement options.
Are flush casement windows worth the extra cost?
On a period or traditionally detailed elevation, usually yes, because the flush line is what makes a replacement look right. On a modern house the difference is slight and the money goes further on the glass.