You usually need planning permission to replace windows in a protected structure, because window replacement is one of the most common works that alters a protected building's character and is generally not exempted development. In practice, planning authorities will often expect repair or like-for-like restoration of original windows in preference to replacement.
Windows are one of the most visible and character-defining features of a historic building, and for that reason, replacing them in a protected structure will almost always require planning permission rather than being treated as exempted development. Original sash windows, glazing bar patterns, glass type, timber sections and even the way windows are set within the reveal all contribute to a building's special interest, so planning authorities scrutinise window proposals closely as part of any application affecting a protected structure.
Conservation guidance generally favours repair over replacement wherever possible. Timber sash windows can often be overhauled, draught-proofed, and had their sash cords or glazing repaired at a fraction of the intervention of full replacement, while retaining historic glass and joinery. Where windows are beyond economical repair, planning authorities typically expect accurate like-for-like replacement matching the original design, materials, glazing pattern and finish, rather than modern uPVC or aluminium units, which are usually resisted because they change the appearance and can damage the historic fabric of the opening.
Because expectations vary by structure and by local authority, it is important to discuss window works with the council's conservation officer before applying, and to consider whether a Section 5 declaration would help confirm whether a very minor or clearly like-for-like repair might not constitute development at all. Carrying out unauthorised window replacement on a protected structure can result in enforcement action requiring reinstatement of the original design.