It very much depends on how the house was owned.
When two or more people own a property it is either held as joint tenants or tenants in common.
If it is owned as joint tenants then all the owners own the whole of it. When one dies ownership automatically passes to the survivors and there is nothing for the deceased to leave to a third party.
If it is owned as tenants in common, then each owner owns a defined share (half, a third, for example) which forms part of their estate and can be left to whoever they like.
Of course there are mortgages and so on to be taken into account as well - a person cannot leave what they don't own.
I have just found this which explains it nicely:
http://tinyurl.com/5u7w8u
You can find out the basis of the ownership by looking at the Registered Title of the property. You can view this at the Land Registry or online.
If it has words like this:
�No disposition by a sole proprietor of the registered estate (except a trust corporation) under which capital money arises is to be registered unless authorised by an order of the court�
they are tenants in common. Without it, it is a joint tenancy