Thank you for your question.
There isn't specific information about flueless gas stoves in building regulations.
The manufacturers do list instructions that must be adhered to for safety and in the event of warranty claims.
The hearth requirements for the Esse FG500 are "This stove must be installed on a non-combustible hearth and be of sufficient size to accommodate the stove."
The stove must be installed in a room larger than 40M3.
In addition it must NOT be installed in a bedroom, bathroom or room with a shower.
It MUST be installed by a GasSafe registered installer.
Full Installation guide is available on the product page in the download section.
Please feel free to contact us with any more queries.