Your point is excellent, and you are absolutely correct in your thinking.
The key factor to consider is the nature of the renovation you have in mind.
If it involves a straightforward renovation where your intention is to incorporate built-in units that will add value to the property even after you've moved out, you can save a significant amount of money by undertaking the project yourself. By this, I don't mean physically cutting the pieces of wood yourself, but rather exploring companies that charge you solely for the unit parts, allowing you to assemble them independently. For instance, in my own house, we are following this approach to save on the costs of hiring a fitter or carpenter. We opted for a company called Jali, where you can design your units online, receive them flat-packed, and assemble them yourself. I have attached a picture showcasing the final outcome.
Several carpenters and fitters quoted me £5000 for the alcove shelving and cupboard project. However, by choosing this company, it cost me less than £800! Consequently, I view this as adding £5000 worth of value to my home while only spending £800, as I painted and assembled it on my own.
Now, if you are considering a larger renovation or even an extension, I would still encourage you not to shy away from doing it yourself. However, in such cases, it may be prudent to have a professional alongside you. Nonetheless, you can always offer your labour on the project to avoid the expense of hiring additional workers.
I hope this information proves helpful to you!