Let's start with bathrooms and kitchens. While well-maintained and nice-looking kitchens and bathrooms do add value to a home, there's nothing to say that they have to be the very latest thing. An older room that is well looked-after and wasn't a complete abomination to begin with can often help the value just as much as a newly-installed one.
Extensions, on the other hand, almost always increase the value of your home, as they make it bigger. A four-bedroom home can be significantly more valuable on the market than a three-bedroom one, as a bigger family can live in it. However, you need to watch out for building too far out into your garden if you only have a small one: get rid of too much of your garden, and you could knock even more value off your home than you gain from the extra bedroom.
Although it might seem strange, this same rule also applies to garden features such as swimming pools and ponds. While you might like them, not everyone does - and you've just taken a piece of garden that could be used for almost anything and turned it into a single-use feature. Also, swimming pools especially can be very expensive to maintain, which can put off potential house buyers.
Overall, it's a bad idea to make home improvements just to make money, as you will almost always fail. Instead, just make the improvements that you want, while avoiding any that will really hit your house's price.