Drainage is put in first on a building site, and landscaping is done last.
Unless this is road or a footpath, no-one is going to be really bothered in altering a cover level
And your two covers are one for foul and one for rain water drainage
drainage is done first. the drainage guys guess at what finished level will be in the garden, then allow another 6 inches for luck and another 12 inches for the crap the builders will throw on the garden before they chuck some crap topsoil on the garden. The landscaper then comes along alters the drains and sorts out everyone elses crap!