That's making it too technical.
Just go look at the specs for the house, what was used before. Reapply.
Designing a new paint job for an unpainted house is a different matter, and may be a more appropriate parallel to consultancy. However, can a consultant be expected to be able to find out how to do something new? And, isn't every business environment somehow unique? At least, depending on the degree of interacting with employees, there's going to be individual differences.
So, ok, you know better, but I think customers might not be able to appreciate the difference between "technically honest" and "lacking in what we need". They may, in fact, not know how to ask the question they want answered.
If I'm asked "can you do this", and I don't know, I'll likely say "Let me have a look, I'll let you know in an hour". I research the process involved and guesstimate if I'm up to the task or not.
