Casement windows are a popular choice, especially if the size of the window opening is important. Price is also a factor, which you can view guides here: upvcwindowsfitted.co.uk/upvc-windows-prices
If you do a search for the best cheap double glazing UK websites, you will be able to find a few companies that manufacture them at very affordable prices.
If you are looking for a double glazing price guide for affordable online glazing prices, you should keep a look out on the Internet for more information.
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they are usually permitted developments, but you will need to comply with building regulations in order not to fall foul of your local council.