New and old structures move differently
An extension has fresh foundations, masonry and finishes beside an older building that has already completed most of its initial settlement. Junctions can therefore be more prone to fine cracking.
Plaster cracks are not the same as structural cracks
A hairline crack along a plasterboard joint or between old and new plaster can be cosmetic. Cracking through masonry, stepped joints or distortion around openings needs a different level of attention.
Drain leaks can matter
A damaged drain beside new foundations can soften or wash out ground. Where cracking is accompanied by drainage problems, both should be investigated.