An alternative to moving, would be to use the space beside the house for a bigger garage and huge bedroom/en suite/dressing room above.

That won't be cheap! Do you get anything like the cost back, on resale, or is it money that basically, will never be seen again?

?

Doesn't it depend on a lot of factors?

For example, it is often said that your location has an impact. There can be a "ceiling price" for an area. So, even if your house has more utility than it's neighbours because of it's extra size, you can't always recup the cash invested.

It's different of course in a sought after area where space is a premium.

Also, check local planning conventions / expectations carefully. It's often much far harder to get approval for two storeys than one (or loft extensions). Certainly, where I used to live in London, two storeys was a no-no (because of the wishes of the residents and not council policy).

Would be a bit tricky for me... in a terraced house.

Whether you get back what you spend depends on how much you spend (how high spec), the market, and the area.