Allen

Hi there I hope someone can advise me if this is possible.... My house was built into a slope. The back facing this slope. I have a single story extension where there was a patio at the back which has an original retaining wall for the garden. The first part of my garden is a decked area which sits at 6ft above the ground level from within the extension. I want to excavate the earth under the decking area and build an underground room and then re pave the area on top. I understand that underground rooms need to be 4m from the house however as this project is at the same level as the house (possibly 1ft lower) can I build this room next to the house?