Traditionally, timber sash windows are the preferred choice in conservation areas because of their unique character. However, if you want to replace them with energy-efficient uPVC windows, you are allowed to do so - as long as their appearance isn’t considerably different from the original design. Before replacing windows in a conservation area, you will need to speak to the local Planning Authority and Building Control, it’s important not to do anything until you have advised them of your plans. If the local Planning Authority have enforced an Article 4 direction, a planning application may be required for each replacement window, as it cannot be done under ‘permitted development’ in the usual way.