Small buildings in gardens or parks where people can relax or have parties in the summer.
Summerhouses are usually seen as pleasant and relaxing places.
Examples:
We had a picnic in the summerhouse.
The children played in the summerhouse all afternoon.
The summerhouse is a nice place to read a book.
The summerhouse is like a little house for the summer.
Our summerhouse is the best place to have a party.
The summerhouse is where we go to relax in the summer.