Hi,
WW2 Air Raid Shelter. I have been advised to inform you of my findings at the end of our garden.
Looks to be quite large, having uncovered the roof! About 12′ x 12′. At one end filled in with earth, but would be most probably stairs entrance.
Interesting. although many domestic gardens had what were known as Anderson Shelters, named after Sir John Anderson – the man in charge of Air Raid Precautions. The dimensions you describe sound about right. Whereabouts are you situated?
Further information on Anderson Shelters can be found at https://andersonshelters.org.uk/ and there’s even a Facebook group Anderson Shelters in your garden.