Hi Ragsta,
The 'short' answer is that you can probably change the door to a window under government "permitted development" rights without requiring planning permission from your Council. The only relevant condition is that the materials that you use for the window must be "of a similar appearance to those used in the construction of the exterior of the existing dwellinghouse".
The 'long' answer (!) is that with PD rights you always have to be careful, because there are a number of "exceptions" to the above "rule". For example:
1) Your property must be a C3 house - if it's a flat, C4 small HMO, or a sui generis large HMO then the above rights won't apply.
2) If your property was built, or converted from another use, from the 1960s onwards, then it may have had its PD rights removed by a condition on the original planning permission (this is fairly common).
3) If your property has been extended from the 1960s onwards, then it may have had its PD rights removed by a condition on the planning permission for the extension (although this is very unlikely).
4) If your property is subject to an Article 4 direction, then some or all of its PD rights will have been removed.
5) If your property is a listed building, then although you would still have the above PD rights, such works would still require an application for listed building consent.
6) You can't combine (as a single operation) PD works with works that are granted PP by the Council. For example, if you get PP for an extension with a door, then you can't built an extension with a window and rely on PD rights for the change of the door to a window.
7) PD rights don't apply to buildings or uses that are unlawful.
Although the above list might look fairly intimidating (!), in practice if your property is a house, was built before the 1960s, and is not subject to an Article 4 direction, then it's highly likely that it has PD rights.
Thanks,
Steve