If the last use of the building was for agricultural storage then yes, you need planning permission to carryout renovations (exactly what are you suggesting to do??). Its somewhat of a grey area, so the safest way to ensure everything is correct is to apply for planning permission. And yes, you need planning to incorporate a septic tank, so a percolation test will be required.
If you are planning on building a new dwelling nearby in the next 10 years, then, assessing the situation using current policies, you may be restricting yourself. Is the location of the site within an area subject to 'Rural Housing Guidelines'? The council may look upon a new application unfavourably, because they may think "you have a house, why do you need another one?? if its not big enough, then just extend".... this is their current mindset. Also, giving you permission for a new dwelling would be absolutely the same as them giving a complete stranger permission, because they cannot restrict you from selling the subject building to anyone. If these restrictions dont apply, then you have to ensure that you have enough site area to take two dwellings, also that a new dwelling doesnt contribute ribbon development, congregation of septic tank etc.. etc.. etc.. ad nauseum....