What is an add-on slab foundation?
There are many older homes where an owner decided to enlarge the foundation by adding to the original foundation. Owners often do this to create a larger master bath area. The are several things you and your clients should know about add-on foundations.
City of Houston requirements
The City of Houston requires that any add-on foundation be designed by a Texas Professional Engineer. The TBPE requires that a Texas Professional Engineer be in responsible charge when a slab-on-ground foundation is modified in any way.
Add-on foundations that may create new problems
There are two situations where an add-on foundation is likely to cause problems: when the two slabs are not tied together and when a patio slab is enclosed and thus becomes an airconditioned living space. Both of these situations may eventually create problems. They are usually minor, but they are clearly not best practices.
Limitations of add-on foundations
No matter how well an add-on foundation is tied to the original foundation, the two foundations are destined to move apart. If piers are used to keep the add-on foundation from tilting away from the original foundation, that will help keep the foundations from moving differentially, but only marginally.
Is an add-on foundation worth doing?
Most people find the distress that results from an add-on foundation to be worth it concerning the value of the added area. The only situations I have seen where there was a serious issue are where there was no attempt to use rebar to mechanically fasten the two foundations together. I once looked at an add-on foundation that had a 6-inch gap between the two foundations. Needless to say, the house was not attractive to the typical buyer.