Windows · 5 min read · Updated 2026
Flush sash vs standard casement windows
It is one detail — whether the opening sash sits flush with the frame or proud of it — and it changes the character of an entire elevation.
The difference in plain English
On a standard casement, the opening part overlaps the frame and stands out by around 15mm. On a flush sash, it sits level, giving one continuous plane, exactly like traditional joinery.
What it costs
Expect roughly 10–20% more than an equivalent standard casement in PVC-U. In aluminium the premium is smaller because most systems are flush by design.
When it is worth it
On cottages, farmhouses, Victorian and Edwardian homes and anything in a conservation area, flush sash is usually the difference between windows that look replaced and windows that look right.
On a 1980s estate house where the original windows were standard casements, the upgrade is far less noticeable — spend the money on glass or colour instead.