Expert: Dan Griffin - 2/3/2008

Question
Can a home owner build a extension of their basement under their backyard?

I am about to add on to my home and would like to add additional basement
under as much of my backyard as possible.
Is this allowed?
Is it costly?
Is there ways to prevent water problems with such a basement?

Thank you in advance for the reply.

Answer
Robert,
Sure, you can have a larger basement within the boundaries and setbacks in your municipality.  You mention several times about a basement under your backyard.  I assume you know that you will destroy the yard, pour the basement, and, then, re-establish whatever yard you create.  Basement space is the cheapest square footage you can add to an addition.  It sounds like you are wanting to add just basement without more house.  This would be a bit unusual, but doable.

YOu will need to be able to get heavy equipment into the back yard.  Get many dump truck loads of dirt out of the back yard and hauled off (unless you can design some decorative berms into your landscaping).  Pump or otherwise deliver a substantial quantity of steel and concrete into the back yard.  Waterproof - not damp proof - the new walls and roof and make provisions for removal of subsurface drainage.  Landscape away and get surface water directed away from your structure(s).

Water problems need to be addressed before construction.  There are waterproofing methods to be used with back up drainage and piping systems.