you are making a gamble on price moves. If, say, it costs you £100k and you sell it for £120k, you have made £20k profit. If prices slump, and it sells for less, you haven't
Worse, if the market stagnates and it doesn't shift, you will have the costs of any loans to bear, as well as maintenance and refurbishment prior to remarketing a couple of years later.
In some places, for example Ireland, Spain and USA, investors hoping to make a profit out of a rising market lost a lot when the reverse happened, and there are still part-built uncompleted developments where the owners ran out of money or the builder went bust. Some went at distressed prices or are now worthless.
If you have enough money to spare some on a gamble, and run the risk of a loss, you might do it, especially if you are experienced in building trades and project management.
Having done it some years ago in a rising market does not mean it will work today.
Building a house for yourself is less of a gamble, because at the end of the day you can still live in it yourself and pay the mortgage, until interest rates rise.