Re.:
Thanks ClubMan!
@ workdude:
Try the usual timber frame companies and see what they can offer.A "special" design according to your plans for timber frame buildings usually need a redesigning by a civil engineer .It should be no problem for any timber frame company to ad to their standard design some extra insulation on the outside.But for a block and brick company it would be resp.it wouldn't make much sense with the cavity wall construction.
Each timber frame company has it's own standard designs,so check them, finding out which standard design would fit you most.As I said,a total new design for a company would lead to considerable higher costs.
Extra costs to be considered beside the extra wall thickness could be a wider foundation,wider windowsills and ,most important and difficult to get here,better doors and windows.Heat recovery systems are ready available.As well as solar panels and small boilers.
"Passive house" means that this house is using less than 10kw/m2/year total heating energy incl. warm domestic water.But these numbers are not agreed as standard,so make absolute sure what the expression "passive house " means.And fix these numbers in the contract.
An important point is the wind proofing,the blower door test is an absolute must.
Passive houses are common standard on the continent,the further north you look the more you see of them.So a foreign company could be of interest.But in Britain they are becoming more common as well,check the web.