Re: Bought 2nd hnd house 3 mths ago: problems with drainage & percolation, any comeba
It is not strictly true that there is never a comeback against the vendor of a second hand property. The 'standard' contract includes a reasonably comprehensive warranty that the property is in compliance with planning permission and building regulations.
A septic tank that doesn't work in rainy weather is, at risk of stating the obvious, clearly not in compliance. Of course if it is pre-'63, there may be no planning condition to comply with and if pre-1992, building regs may not apply. In addition, the 'standard' contract may have been modified with special conditions which to some extent reduce the vendor's duty in this regard.
Nevertheless, the starting point is that a seller who knows of, and fails to disclose, such non-compliance may well have left him\herself open to being sued.
Unfortunately for OP, if the extent of the problem is sufficiently small that remedial works of €3k will solve it, then it is unlikely to be economic to pursue the matter through the courts.