Q about 1930s build

Yes quite likely that air is circulating in this cavity. If there are air bricks round the property, definitely so. The answer is cavity wall fill/insulation. Check by measuring that it IS a cavity wall first of course.

Normal practice pre joist hangers was the either cut holes in the brickwork or to bolt on a wall plate (horizontal bit of wood) and rest/ nail the rafters on that.

They may have been resting on the existing/present wall plate for the house roof.

Yes, the 1930's semi I was dragged up in had 9" solid brick walls. The garage ceiling joists would have been fitted into sockets in the walls, probably gaps left as the wall was built. Have a look at how the ceiling joists fit into the party wall in the loft space.

As for cold extension. When was that built(ish)? Cavity wall above the garage level? Insulated? The orginal garage walls where probably single skin with piers, these may have been demolished and replaced or simple blockwork built inside leaving a cavity. Insulated?

All the other outside walls are cavity, we've had them filled. Above the old garage is now second storey built using studwork [1] so I'd have to take the floor up to check. That may happen, longer term, as I might want to have the CH pipework runs redone for that section.

Extension built about 6 years ago. Original garage wall retained, upper story stud and tile-hung. Oddly, there are some piers but the garage wall was cavity, that was filled a couple of years ago.

We had the floor up in one of the (new) upstairs rooms in the extension, I found I could reach under the floor and touch the boarding on which the tiles are hung - and the sharp ends of the nails that were used to attach the battens on for tiles to hang on. At least in that room I had celotex blocks put between the joists around the edge of the room and fibre-glass under the floor, so it's a bit better now.

:-)

Pretty sure as it seems very thick. Anyway, if it is I could get it filled from the attic.

Thanks - that's helpful. I've got a copy of the plans submitted to the City Council when this recent work was done, I need to check them. I suppose it's possible that may not match as-built, though.

So it's a wooden hut built on top of the garage? What did Building Control ask for as insulation? Or have I misunderstood?

Also what did they do with the original garage wall and presumed concrete floor?

You might be able to research the alteration plans through your building control dept. records.

I was able to find the plans for the flat my wife inherited. 1960's and on micro-fiche but readable.

Still there (it is a cavity wall) and acting as the outer wall for the new ground floor rooms. Upper is stud/tile hung. Concrete floor now has suspended floor above it for the downstairs.

Got those. As I mentioned in reply to harry, I need to examine them now from this PoV.

In message , Tim Streater writes

Hmm.. I don't think a suspended floor over uninsulated concrete goes anywhere near the current thermal requirements.

Sounds as though their building inspector had his eyes shut:-(

Saw that. I like the shed on brick allusion:-)