Bit stuck with getting my head around an issue as to whether to install my heating system myself and forego the RHI. From what I can make out so far because we have stuffed the house full of insulation such that the u values are that of a passive house (but have 2G not 3G) does not look like we would get much RHI and certainly would not cover the cost of a high quality German or Austrian pellet boiler but would possibly cover an Italian job but need to check out if my assumption is correct.
The house is a stone farmhouse but I made the mistake of externally insulating it to early we should have just done enough to get an EPC at the lower end then get the generous RHI and insulate later. Seemingly had we not externally insulated, the house would have been classed as a pre 1900 house the RHI would then have been very generous, but the Green Deal assessor has told me that the EPC would take into account the external insulation which puts the house in a completely different category.
We have added an extension with wider cavities at double the Building regs requirement. We do not have a SAPS because it is not new build and because we have not finished the build cannot get an EPC and the assessor has stated he cannot do one until the heating system is installed, but at the moment I cannot determine the best system to put in. If I could calculate the Specific Heat Demand ( Kwh/sq.m/year) of the completed house then I can use this in an RHI calculator which would give me a good indication of where to go with it. The company that designed the u/f heating system calculated from the u values I have supplied a heating loading of the system in Kw at -3 deg c ambient. Does anyone know how I can get from this figure to the Specific Heating Demand?
Thanks

Thanks steamy tea your first suggestion is something I could do but what is the method? I have the information but not the methodology. The EPC would give me what I want but I cannot get anyone to do it as house ain't finished. Used the calculator on Energy saving trust site but whilst it gives me a guide it is not accurate enough to give me what I want. Can't do the practical bit as house not finished and in particular no insulation in the ground floor yet.

Just a thought - the RHI will be based on the GDA software, which may not necessarily be anything like the same value you'll get by doing a good heat loss calculation. If it were me, I would pay a GDA assessor to come and do the assessment, but not formally submit the result - adding in the missing heating system, or options on it, and see what the outcome might be. I'm sure you could come to an arrangement with the GDA assessor, that when he comes back to do the "official" assessment, there might be a reduction in the fee (a lot of his work having been done the first time).