If you have a storage tank in the roof, then the cold water tap over the kitchen sink is direct off the mains for fresh drinking water and then the rest of the cold water supply is from the tank in the roof.
If you have a storage tank in the roof then the tap over the kitchen sink will be direct off the mains for fresh drinking water and all the other taps, cisterns, baths etc will be via the tank in the roof.
Unless you have a very old house with a cold water tank, when it might supply all the taps. This was done to prevent excessive mains pressure causing a burst pipe or leak inside the home
So there you have it, minihydro, 4 responses, and 4 possible combinations.
DO let us know which of these you have in your house. Was there a particular reason you needed to know?
I don't have a loft tank at all. I've got a combi boiler and the water goes directly from the mains to my taps. I can say this for certain as earlier this year we paid to go off a common supply onto our own supply.
There are four different ways of doing it, as identified by the first 4 answers and they are all valid, so its no big deal. Only in very old houses will the kitchen cold tap be connected via the tank in the loft.
To check your system, simply turn off the mains water at the mains inlet c*ck in the house (if you have one fitted), or on the street, and see if you get a flow from any of the cold taps. Any flow thereafter must be from a storage tank. No flow, and the taps will be direct from the main.