This can be established where the specific scope of works are known from the outset.
Generally between 5% & 6% would be chargeable to provide planning and building regulation services. This would be based on current estimated building costs measured per sq.ft of the new gross internal floor area plus any internal alterations and external works.
Hourly rates are used to establish the parameters of a project with the Client and the Local planning department. Once established, a fee can be agreed based on the percentage of building costs and the hourly time spent to date is deducted from the agreed fee.
In general, using any of these methods work out identically in overall fee due to the type of project, the amount of hours involved and the relative building costs all have a relationship.