When William invaded 'England' there was already an extensive administration that had been in place for centuries.
Each county was sub-divided into smaller and smaller areas, and each of these sub-divisions had its own mini-jurisdiction that accounted for tax-collecting, land-ownership, tithes, etc.
Since these reported back to the next level, these to the next level and so on, the administration of the country was easily the best in Europe at the time.
From what I've read, William looked at it, sensibly decided not to touch it, just made a few changes in laws, taxes as he saw fit.
In essence, the reports from these mini-admin centres were the reason the Domesday Book was able to be compiled when it was.