I will be replacing a septic tank while adding 3 bedrooms to a 3 bedroom house. The existing septic waste pipe is 4″. Will this be enough or do I upgrade to 6″? I think I have 45 fixture units for the house, after the addition is complete. There is plenty of slope in the waste piping.
Your local regulatory authority can answer this question. I would guess a 4 inch house drain should be sufficient. Typically the septic system is designed for the number of bedrooms in a house. If you’re doubling the size of the house, you should also be increasing the size of the secondary treatment system (whatever is after your septic tank). Again your local regulatory folks can give you the best guidance on this, not to mention a permit.