When Bison Frames, the PVC-U window specialist received a request to provide windows that were destined for a grade II listed cottage, the company realised it could be on dangerous territory. In providing a solution, the Yorkshire based company looked long and hard at the existing vertical sliding sash window profiles supplied by Spectus Window Systems with a view to improving the vertical sliding window so that it would stand up to the scrutiny of the local planning officials. The legislation and processes that have to be adhered to when trying to make alterations, however minor, to a listed property.