Underground Bees

[Jjacksh]

Hi, I moved into my house 6 years ago and noticed a small opening in the ground where a large number of bees were flying in and out. They do not bother anyone and have been left alone. They appear to have thrived each year and this year I have noticed a new second opening about 30 ft away under the kitchen window. My first question is what sort of bees are they (some are really quite large)? And do you think the underground hive is connected to the first? I'm having an extension built and the second entrance will be affected. I don't want to endanger the bees, what do I do if this is a new separate colony?

[Littlesparrow]

My first thought was Masonry bees, but they are small. Can you take a picture or provide more information on colour, fluffy etc? Don't know if you can save the bees if you build your extension.