cracks on INTERIOR wall

I have two walls running in the same direction, between 2 bedrooms and a lomnge. Cracks have apearde on both sides of both walls, starting from the outside walll that abutts. They extend about 2 metres each.

I gave spoken to the Ins Co, who say it can't be subsidence, because there are no external cracks. The bungalow was built in the 60's, and these cracks started about 4 years ago, were filled and reappeared a year ago and got worse. It is about 4mm at the worst point, but I can only see the plaster.

Has any one any ideas or observations, please.

Could be the internal walls are built off the floor slab - not with their own footings . Slab is moving slightly within the perimeter outside walls that are solid . Any trees nearby ?

That makes sense. There ARE trees around. What is the remedy? Is it a question of digging under wall and back filling a footing? Would you know if this is likley to be covered under insurance?

I suspect your problem is the same as I have in my bungalow. The concrete floor slab has sunk and the result is cracks in all the internal walls as they try to adjust to the lower, dished floor on which they are now standing.

I had the insurance people out and they said that as the main external walls are unaffected (underpinned many, many years ago) they weren't interested and it was my problem as it affected only the floor slab (there was a specific exclusion in the policy, so it must be fairly common).

My solution is going to be to demolish the internal (brick) walls, dig out the floor, install a suspended wooden floor, replace partitions with stud walls. Lots of work and more costly than other remedies, but given the clay soil here I don't want to be doing a quick fix and then having to do it again in a few years and want a one-off and permanent fix this time. (I'll also take this opportunity to install UFH)

My case is severe (my office chair rolls across the room every time I stand up if I don't have a rug down ) You may have some less disruptive solutions you can consider.

Thank you Mike and Nige, kit does all make sense.

Woud the solution be to use a strongboy (?) to support the wall, dig under wall and backfill to create a footing

OR

The cracks extend from the outer wall to a doorway (on both affected walls. Could we use the strongboys, excavate at the ends of the crack, fill to creat a peir, then use a long lintel?

You don't make it clear where these cracks are or what way they are running, or if they are even or wider at one end

Your description seems to imply that the cracks are at the ends of the walls where they meet the external wall