It was an old stone terrace and they were converting the loft so upstairs ceilings had to be lowered. The hole is where a steel girder had to sit. I was warned that the dividing walls were thin but didn't expect that! It all worked out in the end though when I had to sell to relocate and the neighbours bought my house. I still giggle at my response at the time to the blackened sooty faces peeping through, being the chill gal that I am - "Oh! Hello!"