Loft conversion - when should the house be completely settled?

[Ladyface]

My loft conversion was completed 18 months ago. We had a few hairline cracks come through in the first few months after completion. Then some fine cracks on the first floor. Over the last month or so I’ve noticed cracks on the ground floor. In the kitchen there is a thin crack between the ceiling and wall, a few cracks in the wood around door frames and a couple of vertical cracks in the corners. I don’t know if I’m noticing if more as I’m home all the time. I had to deal with subsidence in my mum’s house so I am paranoid about anything happening to my home. Should the house have settled by now? It’s a 1930s and we used a trusted experienced builder and all the relevant survey and buildings regs.

I'm a little puzzled as to why you would have cracks as a result of the extension downstairs unless you had structural work undertaken around that location as opposed to the loft? Unless the loft conversion has significant additional weight which is bearing in the foundations which in many 1920s houses can be very shallow.

Is it more likely to be a result of a dry period and clay soil giving some minor cracking? Difficult to say but we have a house about 50 years old and have noticed some very minor cracking around the window sealants and ledges and was thinking of this

It does seem as if the weight of the extra floor is pressing down but the timing of this has coincided with very dry weather and we are on clay soil. There has been a lot of disruption over the last few years as before our work was done the house attached to us was completely renovated. Their extension was demolished and rebuilt, loft conversion added and a large leylandi hedge that bordered our back garden was removed. I guess it is a case of waiting to see what happens.

You might find the removal of te Leylandi might have contributed with the clay soil. When removing large trees like that, they suggest it is staged over a period of years to prevent "heave" in the ground which can cause subsistence. This can be exacerbated with dry weather

My BIL was having an extension done. Due to the large leylandi on the boarder which were 6 feet away, he had to have specially reinforced foundations which cost about £8k. He wasn't happy!