Be very careful! … The dimensions of your hearth are covered by law under Part J of the building regs. This basically tells you the dimensions of your hearth from your appliance to any combustible materials (ie skirting boards and carpet.) Plus the definition of the edges. A HETAS registered engineer could turn up and make you / the owner rip your hearth out if its not to spec. Its explained really clearly in Part J which you can get from here ….
http://www.planningportal.gov.uk/uploads/br/BR_PDF_ADJ_2002.pdf
Just had a quick look through this stan, and it seems to be about gas appliances, not real fires,but i will check with a local shop that supplies real fires and wood burners ect.