If you buy a house and it already has tenants in and you want them to stay possibly for the short term but could be long term how much notice would you generally need to give them when you wanted your house back?

Is the house in Canada or the UK?

Canada

Depends I suppose if you are going to draw up a proper lease for them to sign, here you would lay out the terms for giving notice to quit (for both landlord & tenant).
If it is more of an ongoing 'month to month' agreement, then the very least I would expect as a tenant is a months notice.
It is also worth checking if their existing lease is still valid once the property has changed hands into your ownership (I have no idea myself, just a thought!)

Tenancy/landlord issues are a provincial matter, so it depends on which province your house is in. In BC, it is two months, in Alberta it is three. I have no idea about other provinces. It would be a good idea to check with the gov't of the province you're house is in.

In BC: http://www.rto.gov.bc.ca/default.aspx
In AB: http://www.servicealberta.gov.ab.ca/...ds_Tenants.cfm

As Hot Wasabi Peas says each Province is different.

For instance in Manitoba if school age children are in the house the tenant can not be forced to leave during the school year, so you could be looking at upto 9 months notice in some instances.

Thanks to you all for the info - we are exploring so many different avenues at the moment about buying a house in BC and are looking into the possibility of buying and renting out before we actually get to CA.

I asked my first ever tenant to leave with 1mths notice (Alberta). I don't know if that was okay because the rental was on a monthly basis? Or if it wasn't okay at all? Will read HWP's link...

Yes going to look at them tonight - at work at them moment so doing a bit of "skiving" - back to work now though.