Acceptable Ceiling drop

The only way a ceiling is going to be that far out of level is if the upper floor joists have been installed out of level. For example if the joiner just laid the joists on the brickies unlevel wall and never packed them up to level.
Personally I don't think those tollerences the NHBC state are acceptable especially the pipe boxing of 8mm out of square over 200mm. This is why standards are so carp nowadays with tollerences like that. When I learnt my trade tollerences where +/- 1 to 2mm max over the length of the job not 10mm over 2m or 8mm over 200mm that's just way out imho.
Those stated tollerences are just a get out clause for the builder and the NHBC and reflects the carp prices the trades are getting paid on site.
Your ceiling is 25mm out over 4.2m so is technically out above the stated tollerences as the max would be 25mm over 5m or 21mm over 4.2m but with my experience of housebuilders and the NHBC they'll say it's within acceptable tollerences.

 

Slap it up, no one seems to care about levels or tolerances today.

 

Right I thought this was a new build. As its a Victorian house and a refurb you'll have to deal with it and in this case is acceptable. The thing is if the upper floor was flat no-one would check the levels and that's victorian buildings for you. The whole building could be like that especially after the war with all the bombing that went on. It's probably bomb damage settlement.

 

I know mate, unfortunately. That's why I don't do site work and the likes anymore!

 

No because you state in your first post you have a low ceiling and the units need to go up to the ceiling so the builder may have thought of this already. If yo did this you'd probably lose 100mm in height

 

so you've got 6mm/m fall. in my experience rooms are seldom square and you have to make things fit. Think about "halving the error". Set the units so the gap at the top is 0-3mm (which will cover with a bead and be barely noticeable, and set the units 3mm out of true, again barely noticeable. I find one of the tricks with this sort of thing is to work on the basis of "if it looks right, it's right". My kitchen walls are 15mm out of plumb and don't have a right angle corner. I defy anyone to spot this on the installed units and worktops.

 

Try working in Georgian houses

 

Why would he lose 100mm in height? The higher side would just be reduced to the height of the lower side, in this case 25mm.

The OP second link is the correct answer. Use modern products to level the ceiling and there is no excuse for out of plumb ceilings. The steel studs and plasterboard should be DEAD level when a level is offered up.

NHBC tolerances relate to FINISHES e.g plastering. - not that I endorse such tolerances myself.

 

It's not a new build its a refurb of a Victorian house so nothing to do with nhbc. There will not be the nhbc ten year warranty. Did the op actually state to the builder he wants perfectly level ceilings or was the property just bought from a property developer?
If he wants the ceiling level he'll have to do it himself unless it was agreed with the builder before hand.

 

It would be an extremely messy discussion to start whether if a builder was reboarding a ceiling on exisiting timber joists they would be expected to add timber furrings or steel studs etc. to level it out. In hindsight the builder should mention this to the client before commencing works to make them aware.

Regarding NHBC, as with other warranty providers, they do provide warranties on refurbishments not just new build. Question here is would they allow bigger tolerances. - they warranty structure and latent defects, not if a ceiling is out of plumb.