I bought a house (40+yrs old) recently and it turns out the Septic Tank is only 6 feet away from foundation. 

Last year we were plagued with a damp spot that took quite a while to dry up except for high summer. We uncovered it and found old railway ties as a cover with a hole cut about center of the wet spot. The tank is in really good shape but drain field is in question. Neighbors say that is just a pipe leading to a couple of old buried vehicles with a drywell as an overflow. 

Found drywell but loooks like there might be a distribution field there as well but no one has a definate answer. (revious owners passed away)

Right now, the only thing feeding into the tank is the toilet. All the rest of the plumbing feeds into a seperate drywell(?), and this is a very sluggish system. Washing machine dischage can sometimes be too much if runs three load consecutively. 

I would like to build an addition onto the house right where the tank is now.

My questions are;
1) Can I excavate where the tank is now for the new foundation and simply move the tank further away while tying back in to whatever exists for outflow?

2) Should I move simply the tank and put new distribution system in place? With or without 'greywater'?

3) Should I resign myself to a whole new system?

Cost is, as always, an issue. Thank you for your consideration and I look forward to your response.