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Adds 5% to the value of the property - so I imagine what @Soul says is correct. I guess a nicer version would add more than a portacabin
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As @Tenderloin said, we bought the house for £550k in November. 5% (a usual guide estimate) is £27.5k. The office was a shade under £17k.
Even with room for error, I'm comfortable with the fact that the investment has increased the value of the house by as much, if not more than I paid for it.
It's certainly not cheap but it adds similar or slightly higher value to the house immediately and has taken no more than 5 days (not consecutive) to complete. It's also fully compliant with all planning & building codes as well as fully guaranteed for 25 years.
I'd say it's worth the money.