Cannot locate mains water stop cock in my house. Think it was buried. The kichen was moved to an extension in the house but no mains there either. Can I put a valve in the or would the mains be going somewhere else first.

If you are on city/town water then you should have a water meter. The main water shutoff should be just after the water meter (usually on the street side of the building).

If you are like some cities/towns with the water meter outside then you are simply looking for a pipe that comes into your house through the foundation wall (if you have a foundation) or up through the concrete slab... usually found in a mechanical room or coming in under the stairs.

If you still cannot find the main water shut off after that then call your local water department and see if they can guide you to finding the main water shut off valve... it's there somewhere... ;)

Mark

PS: Cold water feeds the water heater so the main water shut off valve will also cut water flow to the water heating device. There should also be a cold water shut off valve going into your water heating device. If this valve is shut at the water heating device your hot water is shut off to the house.