sorry to be the bearer of "it depends" news but it very much depends what you want to do with it
if you touch it for more than a redecoration you are likely to need building control (i.e. if you want to dig down for head height, install plumbing, install new electrics, install further windows or convert it to 'living' as opposed to 'storage') whether you can do some things without building control very much depends on what is there at the moment and you may get away with just doing some work without going through building control and it might all be fine but if in years to come you try to sell a house with a basement bedroom but no building control for the basement it will cause you a major headache
once you start on building control you may find there are several existing things which are fine as they are (or at least were compliant when they were installed) but need to be upgraded as part of the new building control due to new rules, most likely
increased insulation underfloor in the basement - likely to be a minimum of 450mm - that requires digging down half a bloody meter to not gain any head height
the floor slab requirements will also almost certainly be larger than what is there so will require digging up the slab and replacing just to have the same space
there may also be water management and flood management controls to put in place even if they are somewhat overkill
So unless you want a major project then light touch renovation is about as good as it gets, you should assume all basements will have penetrating damp whatever you find at the moment in terms of damp meter so should look at a system to manage the water incoming via a tanking of some sort
tanking render is ok and fine for storage rooms, gyms etc but mostly superceeded now by membrane systems - these are super simple to install and can be done DIY but do require a drain/sump system to deal with any water which does come in and will result in you losing around 4-6 inches from the room size all round (so assume you lose a foot from the room width overall as a worst case) if you can live with that then thats the route to go
I've done full and partial basements in our houses 4 times and it was always worth it but is a far bigger job than a loft conversion as everything you touch seems to create another job
