To save time later perhaps it would be useful for me to point out the following for our readers in the USA:

1 metre is approximately 39.4 inches, so a half metre is within a gnat’s toothpick of 20 inches, depending on the size of gnat.

And a “flat” in Invisibulia is an apartment.

↓ Transcript
1. (hello! what's up?) [I'm digging myself a cellar]
2. (quite an undertaking I should imagine) [yes, it's much harder to do than I thought]
3. (hmm, lots of digging and heavy work?) [indeed, but that's the easy part...]
4. (why's that?) [I started in my kitchen and dug straight down]
5. [it was really easy going after the first half metre]
6. [it turns out that the hard part is dealing with the neighbours in the flat below]