You don't take it with you really, you sell it with the house as most fixtures and fittings, but if that is adding actual value to it...I couldn't say. A greenhouse is not like an additional bedroom or bathroom, not everyone will want one and others might even consider it a hindrance which they would have to pay to take down should they not want it, so I don't know if she could even ask for more money on the house sale for it. It might be a loss she would have to swallow, or pay to have it taken apart and moved to the new property.
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More, more, more. Just need more. One more dress, one more ring, one more car, one more house and THEN I'll be happy.
Soon enough she'll have to add one more husband to that list, I think.
How sad.