I think this is a summer house. It’s situated near Kjósarskarðsvegur (highway no. 48) between the Þingvellir road and Hvalfjörður.
This route makes for a lovely drive in the summer and a spectacular one in the winter when the ground is covered in snow and there is a clear blue sky overhead. It does go over a mountain pass, so make sure your car is properly equipped before you set off, because there can be snowdrifts on the road.