1193 digging a septic tank

We need to install a septic tank.
We need to go under the narrow village road and then dig a hole in our garden for installation.
Our problems are
1.Do we need permission to bury tube under road?
2.Because the garden is on a slope and the soil isn"t too deep a freind has suggested we may start a land slide if the rain water gets under and at the side of the tank.
3.As we are also installing a gas container-can we use the same channel under the road to put different tubes and pipes in?(We also need an electric cable to provide lighting etc down in the garden)
If we do need permission does anyone know how long it should take to come through?
Thanks-apples