cracks in new-ish house

[novice11]

I bought a new Persimmon house 4 yrs ago and soon after, hairline cracks started to appear in the plaster above all doors, some barely visible, others quite clear and they are still developping now. The garage door between the kitchen and the garage proper is particulary bad, it's cracked all around the frame. Is it the wood expanding and contracting ?
Also, where the dormer in the master bedroom meets the wall, there's a long horizontal crack, max. 2 mil wide, and a bit of a steel bracket is even visible. I suspect it is all superficial but what do you reckon ?

 

[masona]

Most of new built home have a 10yrs warranty.

Ten Year NHBC Cover
The industry regulated 10 year warranty cover is applicable to every Persimmon home. A comprehensive guide explaining the cover will be supplied with your Masterfile.

 

[HandyJon]

...hairline cracks started to appear in the plaster above all doors... Also, where the dormer in the master bedroom meets the wall, there's a long horizontal crack, max. 2 mil wide

The cracks above the doors are most likely to be due to the house drying out and settling. All new houses will have such minor cracks.

The dormer crack could be due to something else though since 2mm is quite a bit for a drying out crack, especially since it the junction of structural elements.