hairline cracks in external plasterwork

[villa 1]

I've bought a new house in the past year and I have recently noticed long hairline horizontal cracks and some shorter vertical cracks at different heights in the external plasterwork. These cracks whilst barely vivsible seem to open slightly more in hot dry weather.
What is causing these cracks to appear? My neighbour in the adjoinmg house seems to have similar cracks. There are no cracks on the internal plasterwork.
Should I contact the builder with a view to repair or just paint over.

 

[use2b]

These just seem to be settling cracks - every house will have them - however, if the same crack appears in every house in the same place on each house then there might be a case for getting the builder back to take a look

 

[wertyu12]

Settling cracks is what they are but if there are coming into the ceiling also then you would need the builder to take a look as it could be something to do with the structure.