Here are the completed sentences:

How long have they had the house? (This is the most natural way to ask about how long something has been owned)

The broken vase couldn't be repaired by him. (Passive voice construction)

The garden still needs to be dug. (More natural sentence structure)

Is there a cheaper carpet than this one? (Improved clarity)

Now I don't go climbing as much as I used to when I was younger. (Rewritten to emphasize the present situation)

There used to be dreadful fogs in England, but not anymore. (Highlights the past and present state)

We haven't decorated this room in years. (More natural way to express the time that has passed)