How long will an extension take to settle?

We had a kitchen extension built at the back of our Victorian the house some 6 years ago. The last couple of years a number of hairline cracks appeared in various places at that end of the house, which we attributed to settlement of the extension (and the particularly dry summers we were experiencing). These cracks have been mainly in my daughter's room (the kitchen is below) and the bathroom which is adjacent to her room. Since then worryingly bigger cracks have appeared in our bathroom (horizontally and vertically), the grout has cracked down the corners of the room, around the windowsill etc and all around the walls towards the ceiling.
We asked a local surveyor to come and look and he said it was to do with the fact that one particular wall in the bathroom wasn't well supported as the joist ran the wrong way underfloor (the bathroom is a triangular shape - cut out from another room at some stage in the house's life). He told us to move the heavy furniture away from the wall on the other side (in my daughter's room) - which we did, but the cracks have got worse.
On top of all this cracks have now appeared all the way up the stairs where the wood skirt is now coming away from the wall, the dado rails are moving slightly too. The ceiling downstairs is showing lots of new cracks....I could go on and on....and I thought it was all to do (initially) with settlement of the extension. The whole bloomin' house seems to be on the move.
We want to redecorate our daughter's room but I'm loathed to start because it's bound to crack again as soon as we have decorated.
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The extension will settle within a few seasons say 2 years.
In anycase, it should settle evenly, so that you don't get some parts settling more than others and cracking.
Are these really settlement cracks or seasonal movement or normal expansion and contraction. Settlement is a specific condition, and will show tell-tale cracks.
The cause could be one of a number for other things, so the diagnosis needs to be correct.
The surveyors comment on the floor/wall support does not seem to explain all the other cracking.
dg
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dg wrote:

My guess is the foundations are crap.
It sounds horribly like early signs of subsidence due to shallow foundations and nearby trees.
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wrote:

Shouldn't be....the house was underpinned in 1983
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Jo wrote:

But that would not do much for the extension as that was built 17-18 years later. The fact that the house needed underpinning suggests there is some movement in the land, and if that was not accounted for in the foundations of the extension, that could be a problem.
Why did the house need underpinning? What area is it in? Are there big trees nearby? What type of foundation was uses for the extension?
--
Paul Matthews
snipped-for-privacy@cattytown.me.uk
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Paul Matthews wrote:

I meant to type all that in, but couldn't be bothered..
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The house was underpinned as a result of a tree growing too close. We are in Leigh-on-Sea in Essex - known for movement! A structural surveyor has indicated that the tree - which is still there - should no longer be causing a problem. The foundations for the extension were deeper than the original founds.
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Jo wrote:

Were they approved by the BCO?
You may have to go down > 1.5 meters for stability in some soils (clay is worst) with nearby trees.
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Checked the plans. The foundations were deifinitely approved and they were a minimum of 550mm below root growth.
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Jo wrote:

Well teh other possibility is that the extesnipn wasn't tied onto the house properly, and its just moved away slightly due to expansion/contraction.frost getting in the crack between...
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Mmm....possible. We are built on clay soil so that could be a reason, although I would have thought that with any signs of subsidence there would be racks on the outside of the building.
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Jo wrote:

But was the extension? ;-)
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