Legislation does not allow the installation of a septic tank any more. You'll have to have a treatment plant of some sort, sometimes called a digester.
In haste : search for ( here and elsewhere on Tinternet) 'digester' , 'Approved Document H', 'off mains drainage ', and finally 'General Binding Rules'
A local landowner ( of the field next to us) got as far as building a shepherds hut and putting it on his field, but when he did his Due Diligence about renting it, he found the foul drainage issue too costly, and so the hut now sits in his workshop yard. His point was that renters expect flushing toilets and efficient showers.
A Chuck-It-Bucket doesn't cut it ( his words, not mine - sorry - its Friday 😑)