It is normal for a client and his consulting engineer to have a formal agreement covering the services to be provided and the fee basis for these.
There are a number of different methods of remunerating a consulting engineer for his services, based either on the actual or estimated time spent by his engineers on a project or on a percentage of the total contract value.
The method selected and the accuracy with which consulting engineers can forecast their fees is very dependent upon how precisely the scope of services has been defined. An indicative fee estimate is not a competitive bid.