ya in clare its something similar. Wheras a couple of years ago an extension application was straight forward and no problems, now its becoming something of a pain. i have to advise people that they may get caught in this web if they apply for an extension.
The council will argue that the extension is extra development and hence more occupants are possible. Especially if there are extra bedrooms involved. And even if u argue that u wont be having any more children etcc, they will say what if u sell the house, or they will say that this applicattion may just be an attempt to increase the value of the house. The problem here is that even if u establish that the existing septic tank and percolation area were designed and constructed properly and to the relevant standards at the time, the standards have now changed and more area (usually) is required to accomodate the percolation areas of today.
Your problem though is an unusual one. But if u currently have no percolation area and the septic tank is discharging directly to ground/soakpit (which is terrible by todays standards), maybe you could come to an arrangement with the Co. Co. to improve this situation. This would be in ur interest also (who wants to be responsible for polluting the local water supply, or see sludge bubbling up thru' the ground in wet weather in winter time or have a pet or child infected/die as a result of this.)
Obviously if the Co Co have any common sense at all they will go with u on this. I mean wouldnt anyone agree that if u improve the situation, its better than leaving as it is.
Improvements could include a polishing filter with relaxations on the distance requirements. The current Epa 2000 document is only a guideline and has not been enshrined into the Building regualtions as far as i know.