Often yes, because designation as an Architectural Conservation Area (ACA) can remove the normal exemption for minor works such as window replacement, particularly on elevations facing a public road or street. Whether permission is needed depends on the specific ACA and the elevation involved, so it should be checked with the local authority before work starts.
In an ordinary house outside any special designation, replacing windows is usually exempted development under Schedule 2 of the Planning and Development Regulations 2001, provided the new windows are broadly similar in character to the existing ones. Once a property is within a designated Architectural Conservation Area (ACA), however, the Planning and Development Act 2000 allows planning authorities to restrict this exemption, and many Development Plans specifically remove the exemption for external works, including window replacement, on elevations that front onto a street or are visible from a public place.
Whether a specific window replacement needs permission depends on the wording of the ACA designation and its accompanying character statement, which will often set out what features, such as timber sash windows, glazing patterns, or historic shopfronts, are considered to contribute to the area's special character. Works to elevations not visible from public areas, such as a rear extension window, may sometimes remain exempt even within an ACA, but this varies by local authority and should not be assumed.
Because getting this wrong can lead to enforcement proceedings requiring reinstatement of the original windows, homeowners in ACAs planning to replace windows should check the relevant Development Plan provisions, review the ACA character statement, and, where there is doubt, either apply for planning permission or request a Section 5 declaration confirming whether the specific works are exempted development.