Guide to standard casement window sizes
Whether you’re replacing existing windows or looking to install new windows in your home, measuring the size correctly is the crucial first step in the process.
Please note that the window sizes and building regulations were last updated in this article in January, 2026. Please refer to the product specifications for the most accurate size guide.
Click on one of the sections below to learn more:
Standard sizes and positions for casement windows
Casement windows are the most popular style of window. These are hung on the side or top of a frame and can open in or out. In some cases, they don’t open, and these are called ‘dummy sashes’ or fixed windows.
Read more in Origin’s full guide to casement windows.
There are no industry-standard sizes for casement windows; this is because the design and age of the property vary significantly. Despite this, some manufacturers offer ready-made windows in a range of set sizes.
These won’t be an exact fit for every aperture, but they will be near enough in size so that they can be installed with some adjustments to the surrounding masonry and brickwork.
Although there’s no standard size for windows, there are some basic ‘rules’ for how they’re positioned, which can help dictate the size of the windows themselves:
· The top of the windows is in line with the top of the doors in the room.
· The bottom of the window cill is 914mm (approximately three feet) above the floor.
However, the trend for larger windows, including floor-to-ceiling windows, means that the guidelines above don’t always apply. It's also important to consider the energy efficiency of your windows if you're opting for large, bespoke windows. Features like double glazing and thermal breaks are essential.
Window size considerations in different positions?
While there are no set standard window dimensions, certain building regulations and practical guidelines can help you decide how large to specify your windows.
· Ground floor: Modern trends are increasingly favouring large ground floor windows, especially floor-to-ceiling ones. If you install windows lower than 800mm from floor level, or within 300mm of a door up to a height of 1500mm, building regulations require the use of toughened safety or laminated glass.
· Level 1: For first-floor windows, size and style often balance light with safety. Upstairs window sizes vary significantly, depending on how much light you require. Home offices may benefit from larger windows; meanwhile for bathrooms or bedroom windows, where privacy is required, moderately-sized apertures are usually suitable (provided they meet emergency exit regulations).
· Level 2 and up: In multi-storey buildings or flats, tilt and turn windows can be a practical choice that balances natural light, safety and practicality.
· Bathrooms: While bathroom windows aren't required by law, there are ventilation requirements. Average window sizes are often smaller for privacy reasons, but bathroom windows still need to meet modern performance requirements, and this includes emergency exit requirements and the need for trickle vents.
· Emergency exit windows: Also known as egress windows, these are a legal requirement under building regulations to provide a means of escape in the event of a fire. Each room must have at least one egress window that meets three specific criteria: a minimum width or height of 450mm, a minimum openable area of 0.33m², and a cill height no more than 1100mm above the floor.
Comparing the most common standard sizes by window type
Many window manufacturers, including Origin, only offer bespoke window sizes, but this section compares the size possibilities with different configurations and type of windows:
· Casement windows: As the most versatile window style in the UK, casement windows are highly versatile and are available in numerous configurations. We explore the possible sizes in the section below.
· French windows: This design allows two sashes to open from the middle and the central mullion moves with the sashes, providing an uninterrupted view. As a set configuration, the range of sizes are between 0.47m x 0.47m and 1.8m x 1.4m.
Compare French and casement windows here.
· Bay window: These windows project outward from the exterior wall, creating a sense of space and a focal point for the property. They can be configured as a combination of fixed frame windows (Origin’s max size is 4800mm height/width or 7m2) and casements (Origin’s max size is 1000mm wide x 1800mm high for side hung casements).
· Tilt and turn windows: These windows offer unique ventilation control by tilting from the top or opening inward. Standard industry single units typically range from a minimum of 600mm x 500mm upwards.
· Sash windows: Traditional sliding sash window sizes are moderate (the industry average is around 1.2m tall). However, contemporary designs can be bigger which is ideal for modern homes.
How does window frame material affect size?
The material used for the window frame significantly influences the maximum possible size of the glass panes and the overall aesthetic.
· Aluminium: Due to its inherent strength, aluminium windows allow for much slimmer frames and significantly larger expanses of glass than some other materials. This makes it the ideal material for creating the impression of the largest windows and maximising glass and light, creating a sleek overall look.
· uPVC windows: Highly popular but not as strong as aluminium, so it is less commonly used for expansive windows. Also, due to being a bulkier material, uPVC may not maximise your window aperture as efficiently as stronger materials.
· Timber: Timber windows are frequently custom-made, as they are often required for period properties or conservation areas. However, the heavy, bulky material doesn't easily lend itself to expansive views, like aluminium does. Its maximum views are often smaller than aluminium’s, but it does offer a warming aesthetic.
Compare window materials here to decide which is right for you.
How high does a window need to be off the floor?
Again, there’s no set standard for this, but if you’re looking to install windows which are lower than three feet, you’ll need to make sure that the panes are glazed with toughened safety glass or laminated glass.
Building regulations for UK homes state that windows should feature safety glass if they’re to be fitted in the following positions:
· Below 800mm from floor level
· 300mm or less from a door and up to 1500mm from floor level
As with all windows in a family home, you can have your windows made with restrictor hooks that prevent the window opening wider than 10cm, to ensure children can’t fall out.
Ready-made vs. bespoke windows
Find your local installer today to bring your vision to life
Ready-made windows
Retailers of ready-made windows only sell windows in common sizes. These may not be suitable for your home if, for example, you need a particularly large or small window or a floor-to-ceiling window.
Buying ready-made windows is usually more affordable than bespoke. However, you’re likely to incur costs when installing the windows as you’ll need to make adjustments to ensure the windows fit properly in their apertures.
These additional costs can include:
· Scaffolding hire
· Insulation
· Expanding foam
· Brickwork and accessories
Bespoke casement windows fitted in a barn conversion
Bespoke windows
Although more expensive than ready-made windows, bespoke windows are made to measure. This means an installer will measure, order and install your windows for a guaranteed fit.
View the bespoke case study here.
At Origin, we manufacture bespoke windows so you can have the exact size and configuration you need. Once you’ve chosen the window style and configuration, your chosen local trade partner will visit your home to measure the aperture, suggest appropriate sizes and answer any questions you may have.
Our aluminium windows are available in bespoke measurements up to the following dimensions: For more information about the sizes available for Origin’s casement windows, read our window size guidelines.
| Window type | Maximum width | Maximum height |
|---|---|---|
Top-hung casement windows | 1500mm | 1500mm |
Side-hung casement windows | 1000mm | 1800mm |
For more information about the sizes available for Origin’s casement windows, read our window size guidelines.
Bespoke casement windows from Origin
How to measure casement window size
Why deduct 10mm?
A manufacturer will choose the smallest measurements for your windows to enable the installer to use spacers and fillers to fit the window tightly into the aperture.
If a window frame is too big for the aperture, this can lead to added expense and delays as the brickwork around the frame will need to be adjusted significantly to accommodate the window.
If you prefer the DIY approach, follow our advice below to get the most accurate measurements:
1. Measure in millimetres to get the most accurate size.
2. Measure the width and height from the edge of the brickwork opening that the frame sits in from inside and outside your home. Your measurements should include the cill and frame extension.
3. Take measurements from three different places across the height and width of the frame. The manufacturer will use the smallest measurement to produce your windows.
4. Deduct 10mm from each of your measurements.
Please note – it is always best for a professional to survey and measure your windows. If you choose Origin as your manufacturer, your installer will be the one measuring, ordering and dealing with Origin directly.
Casement windows from Origin, including a central fixed window
Planning permission considerations
Replacing existing windows or installing new windows does not usually require planning permission as this work is classed as a permitted development. However, there are some exceptions.
You usually won’t need planning permission to install new windows that are larger than the existing ones, unless they’re significantly bigger. No matter what size windows you’re installing, you’ll need to follow building regulations guidance, as this ensures your new windows meet a certain standard in terms of:
· thermal performance (consider double or triple glazing)
· safety glazing
· safety and locking requirements
· ventilation
You can find out more about planning permission and building regulations for doors and windows on the government’s Planning Portal website.
Floor to ceiling windows in a barn conversion
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What size casement windows meet egress guidelines?
Certain windows in your home will need to open wide enough to allow people to escape if there’s a fire. These are called egress windows, and are required by building regulations to ensure they “provide at least the same potential for escape as the window it replaces”. The required number of egress windows is one per room.
Egress windows need to meet the following criteria:
· Either the width or height are not less than 450mm
· An openable area of a minimum of 0.33m²
· The bottom of the openable area of the window is a maximum of 1100mm above the floor
Find out more about the specifications for egress windows by reading our guide.
Architecture terms
Having windows measured and replaced means you may encounter some industry terms that might be confusing. These are explained below:
· Mullion
The vertical part of a window frame. A mullion divides the different sashes or fixed windows while providing structural support.
· Transom
The horizontal part of a window frame that divides the different sashes or fixed windows.
· Flying (or floating) mullion window
A window split into two casement sashes that both open from the middle outwards. When open, the mullion moves with one of the sashes, so there’s no central frame (mullion), meaning the view is uninterrupted.
Read more about the specific terms used in window manufacturing in our windows glossary.
