I’ll assume you own the land. You must have planning permission from the local Council. You will also need Building Regulation approval to prove you have met the requirements for installing a septic tank, because in addition to the tank, as you say, you’ll need a soakaway or a drainage field or a small sewage treatment plant for the waste water.
The regs have got tighter recently but your Council website should have information and links to the relevant environment agency website.
Hope that helps.
some people would just do it, without taking shortcuts though as to design and discharge and not tell anyone. so long of course that it is their own land and there is no possibility of leeching into water courses, which is where the proper design and installation come into play .
to put that into context, if you decided to dig a hole and bury your toilet waste on your own land, it wouldnt be illegal .....
https://www.gov.uk/permits-you-need-for-septic-tanks
The above link to the UK Gov regulations may also be helpful as it may help you decide whether or not you need a permit for England, and includes a link for the Scottish regulations.
It seems that if you are in Scotland, you DO need a permit.