I've been toying with enclosing our back covered deck and bumping out the house for a number of years. I had talked to my sister and brother in law (who are both architects) and they suggested a rule of thumb was $100/sq foot. They are in Vancouver so costs may vary but I think it's a reasonable guide.
The other thought I'd have on whether to do the work on not is how long you plan to be in the house. If the Reno improves your enjoyment of the house and would result in you staying there for a long time then I don't think it really matters too much. If it's move vs reno and the reno means you'll stick in the house for 10 years then might as well do it I'd say. If it wouldn't really change how long you'll live in the house then probably better to spend the money on freshening everything up.
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