Extension - floor between extension and house

devil84 · 11 Feb 2026 at 23:58

Hi Just having extension done and build is done. Extension base is concrete and house is block and beam

There is a cavity hole between extension and house flooring. What is normal process for the cavity hole to be filled. It’s over air bricks so builder said they cant just fill it. They just put a “cavity cover” on it. Just basically a white looking sheet.

However flooring people saying they can’t screed over the cavity cover. Any thoughts or advice

DB_SamX · 14 Feb 2026 at 22:00

Do you know why does the extension have a concrete base when the house has a suspended beam and block floor? Usually the latter is there for a reason e.g. poor ground conditions, or ground gas or as flood prevention, etc. Has Building Control signed off on the slab?

(Asking as a structural engineer)

My memory is vague to the specifics and perhaps there are exceptions, but I think you're still required to maintain ventilation to any elevation even when an extension is built, to ensure ventilation and prevent damp under the house.

Peerzy · 15 Feb 2026 at 23:19

We had an extension done 2 years ago, main house is mostly beam and the extension was concrete slab. They should just 'extend' the air brick from the existing house to the exterior of the new extension using some plastic tunnel things and then concrete and screed over it all.

I'm not sure why you'd keep the cavity at all, it would mean getting the floors level would be much harder and make screeding a nightmare. I would expect them to extend the airbrick vent and then fill in the cavity unless it's an unusual build.

DB_SamX · 17 Feb 2026 at 22:22

Peerzy said:
We had an extension done 2 years ago, main house is mostly beam and the extension was concrete slab. They should just 'extend' the air brick from the existing house to the exterior of the new extension using some plastic tunnel things and then concrete and screed over it all.

I'm not sure why you'd keep the cavity at all, it would mean getting the floors level would be much harder and make screeding a nightmare. I would expect them to extend the airbrick vent and then fill in the cavity unless it's an unusual build.

Yeah, good to see the venting maintained here.