We are doing a huge extension now, which technically we project will make a loss, or just break even on upon completion....However, we aren't doing it for the profit- and other houses that are in the same cost bracket in our area are actually far smaller, older, and nowhere near as 'done' as ours will be- not by a mile- you'd have to spend loads on a new kitchen, paint/carpet/plastering/new windows/doors- literally everything.
Additionally, this place has a massive garden and loads of parking compared to other places in my area.
Instead of mortgaging a big house, we bought cheap (ish) last year, and have grafted and saved to pay for the majority of the extension in cash- so in this way we have saved money as we aren't borrowing loads on a mortgage to buy a bigger (ish) house, or paying stamp duty and solicitors fees.
This way, we are getting exactly what we want- massive kitchen, utility room, ensuite, walk in wardrobe, 4 massive double bedrooms, a study and a gym....
So, yes, it will possibly look like a financial loss initially, but as PP said, in 5-10 years we will be way in profit, with a lush, totally 'done' house that meets all our needs, with loads of square footage and other features that the others just don't have in this area.