Tree wasp, Dolichovespula spp

Adult worker wasps are between 15 - 20 mm long, they have a black and yellow "striped" colouring of the abdomen with a black head and thorax.

Wasps are colonial creatures and live in large social groups. The colony is established by a queen in spring and tends to grow rapidly from that point. Many thousands of wasps can reside in a single nest / among a single colony, living on sweet-materials and flower-nectar.

Toward the end of the summer, the colony develops males to mate with the queens and restart the colonising process. Dolichovespula spp usually produce grey aerial nests, which have a regular laminar construction. They may be suspended from trees, bee hives, nest boxes, dormer windows and rafters. If nests are formed underground, the envelope is always visible.
 
When present in large numbers (which is often the case), and if the nest is located where human contact is possible, wasps can inflict painful (and potentially very dangerous) stings.

Treatment

Wasps nests are usually treated by dusting/spraying insecticide  into the nest and around the entrance. Specific indoor sprays can also be used for controlling wasps in localised areas.