Flush Casement vs Stormproof

The Key Difference

The difference is simple: when the window is closed, does the opening sash sit flush with the outer frame, or does it overlap the frame?

• Flush casement — the sash closes into the frame so the front face is completely flat. Clean, minimal, elegant. This is the older design, used since the Tudor period.
• Stormproof casement — the sash overlaps the outer frame by 10–15mm when closed, creating a visible step. Better weather resistance. This design became standard from the 1930s onward.

Both are casement windows — hinged at the side, opening outward. The difference is purely in how the sash meets the frame.

Which Properties Suit Which Style?

• Flush casement — cottages, farmhouses, Arts and Crafts houses, Georgian townhouses, Victorian villas (non-sash elevations), barn conversions, and any property in a conservation area where the originals were flush. Planning officers almost always require flush casements on pre-1930 properties.
• Stormproof casement — 1930s–1960s semis and terraces, post-war housing, modern extensions, and new builds. If your property already has stormproof casements, replacements should match.

Performance Comparison

• Weather resistance — stormproof has a slight edge because the overlap creates a double barrier against driving rain. However, modern flush casements with compression seals and proper drainage perform excellently in all UK conditions.
• Thermal performance — identical. Both accept the same glazing units (double or triple) and achieve the same U-values.
• Security — identical. Both use PAS24 multi-point espagnolette locking.
• Sightlines — flush casements have cleaner, slimmer sightlines because there is no overlap step. This maximises the visible glass area.

Cost Difference

Flush casements are slightly more expensive to manufacture because the tolerances are tighter — the sash must fit precisely into the frame with no visible gap. Expect approximately 5–10% more than stormproof for the same size and specification. Use our configurator to compare exact prices.

Conservation Area Rules

If your property is in a conservation area or is a listed building, the style of casement window is not a free choice. Planning officers will typically require you to match the original window type. Pre-1930 properties almost always had flush casements. Installing stormproof windows on a Georgian cottage would likely be refused.

We work with conservation officers regularly and can advise on what will and will not be approved. Contact us before you apply for planning.

Our Recommendation

If your property is pre-1930 or in a conservation area: flush casement. It is the historically correct choice and will not cause planning problems.

If your property is 1930s or later and not in a conservation area: either works. Choose flush for a cleaner look, stormproof for slightly better weather protection and lower cost.