Generally we work to the RIBA (Royal Institute of British Architects) scale of fees, which loosely ties the architec's fees to the value of the overall project on a sliding scale.
We are a small consultancy and as such have recently taken the decision to de-register for VAT. Increasingly the majority of our clients domestic and not business, we felt that we should at least allow our clients to avoid taxes on the Architectural aspects of their building project.
Unfortunately current HMRC policy is that for the building work itself, whilst new build projects are VAT exempt, renovation and conversion building work is generally not. There are notable exemptions for example conversions to accommodate disabilities in the home.
HMRC guidance for this is available here : VAT for building work.