the rule of thumb is 1inch of steel for every 12 inches in lenth of wall to be removed and suported of the 4 inch wall if it was nine inches thick it would be 3 feet opening would be 4x4 with six inch on each side sitting on concret pads all with in building regs
the main reasion osb boards are used if fire regs for lofts ply will burn much quicker were osb board takes longer every one knows when it is wet its just like weaterbix if for the fire regs
you will get a gas safty cert from any gas safe listed ,price will cost you any thing from £35 to £85 depending on were you live and the company you call just ask for a gas safe cert it make no diferance who fitted the boiler they only check the gas is safe
you will need to seal the stack off before you fit the burner with a fire proff board you can get one from any building suppliers about £10 ,you will then have to cut a round whole the same size of your flue the T is a peace of flue pipe with a cap on it so you can have your chimney swept other...
did they not put the flue in with the aid of a camra bost stacks will take a 7 or 8 inch flue which is best for log burners if you fit a 5 inch you will always be getting it swept so fit a T for excess to save having to remove the burner out every time you get it swept it may be your...
as long as the flue is clean and fire proof and you seal the stack from the pipe and I would put a T in before you seal it off as you will need excess to sweep the flue I have fitted quite a few
if you can give me more info I would be happy to give you a rough quote for the cost you will be paying out I have been in the property refurbisment for over 40 years and run a small family property maintenance company a straight up and over roof with new battons and felt one stack flashings...
if you are thinking of building up over your wall it would be best to fit some steel above then put your stud wall under the steel ,then if you decide to remove the wall you can and it will save you time and money in the long run I have been in property refurbishments for over 40 years good...
you can render over the insulation boards you will need to put a mesh over first you can get the render ready to use in 25 liter buckets but dont forget to vent each wall