If you cannot deal with your wastewater properly then you do not have a building site. If you cannot connect to the mains wastewater network, then you will need an onsite wastewater treatment system (OWWTS) including a percolation area for effluent disposal to ground. In NI, wastewater systems may also be approved by NIEA to discharge to a suitable waterway if one is available.
Even if you have a compost toilet (dry toilet system that does not require flushing, which many planning authorities don’t view favourably) you still need a way to deal with greywater (dirty water from showers, machines, etc.). Also know that some local authorities will not view a compost toilet as an adequate means of dealing with wastewater, as a good composting toilet keeps urine separate from solid waste and uses no water for flushing.