I have been looking through posts on this site regarding CfSH and thought I would share my experience and some advice which hopefully people will find useful.
I am an architect and in the process of building my own house. I am a big fan of the CfSH in principle as it takes such a broad outlook on a property / project, something which all other standards seem to fail in. When looking at finding an assessor I was a bit taken aback by the price quotes, and consequently undertook the training myself to become an assessor as it would also be another string to my bow for my architectural practice http://www.pa-ad.co.uk/.
Through the training and my experience thus far it has become blatantly obvious that CfSH is great in principal but not in practice. I myself am going for level 6 on my project (http://the-old-lemonade-factory.blogspot.com/) but the extra cost involved is quite shocking, hence the reason it will never become mainstream. There are benefits involved such as reduced mortgage rates and exemption from stamp duty, as well as the personal satisfaction, a practical efficient and green home, but these do not help with the upfront costs involved.
I would say that levels 3 & 4 are reasonably easy to achieve, especially if you are going for some sort of ecohome, but 5 & 6 are hard to achieve and will come at considerable financial cost. An essential piece of advice would be to get someone involved who are competent with and have a good knowledge of CfSH at concept / planning stage. Leaving the assessment to the end could leave you with lots of stressful and financial headaches. It will also save you money, as you will be able to pre determine where you can score the amount of points you require and how do this economically - to revisit this afterwards can be very costly and extend the project time considerably.
I would also suggest that serious thought be given to why you want a CfSH rating as it is costly and with the higher levels quite restricting on how you design and build your home. I would even go as far as saying that unless a site is suited towards the CfSH criteria, it may be very hard to achieve a level 5 and impossible to get a level 6.
I hope people will find this info of some use.