Building Homes or Extensions/Basement addition

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Expert: - 1/29/2011


Question
Sir,
I live in northern new york and i live in a ranch style modular home. the home sits on a concrete slab with a 3 foot crawl space inbetween it. i got in contact with the manufacture of the home and he sent me the blueprints of the house. he told me that the house can have a basement under it. how would i go about putting a basement in? what would be a rough estimate of the cost. almost all of the homes in my neighborhood have basements. how much of the work can i do myself and save on costs. i am not afraid of doing it myself but i do want it done right! thanks.

Answer

undermine
Hi David, this is a daunting task your are attempting to undertake.  I'm not saying it can't be done but you must realize that, depending on the method, it may take months to complete.  That being said, I am going to outline several possibile ways to go about this.

The quickest way to do it would involve contracting a house moving company to come in and either "jack" the house up high enough to get equipment and manpower underneath to remove the old slab and whatever foundation walls you have existing. Or have them move the house off to the side until the time comes for them to replace it on the new foundation.  If you have a place nearby you can stay while this work is being done you won't have to worry about hooking up your water, sewer and electric temporarily while the house foundation is being modified.  With this type of operation you can bring in heavy equipment, backhoes and bobcats and in a couple of days have the old slab removed and the new hole dug.  Where you live there is probably at least a 36" depth requirement for your new wall footings due to frost line.  But you will probably want to go at least five feet deep with your new foundation or deeper depending on how much "stem wall" you want above the finished ground elevation.  At this point you can go one of two ways.  You can do the entire wall out of masonry or concrete or you can do a partial concrete or masonry wall topped with a wood framed wall for above grade.  I mention the wood framed wall because the moving company will need to set the house back in position while it is still on their beams and the foundation will have to be done so that they can remove the beams once the house is set back down.  Then the beam holes are filled in.  

Digging out a foundation by hand while the house remains in position is not an impossibility but in your case could come close to being impossible because of the concrete slab underneath.  This slab has to be removed and doing that in a 2' crawl space area can be extremely difficult.  For a project like this you would want to have a large ramp excavated at one end of the house so that you can work your way under the house through the end that has the least amount of weight bearing and preferably the least amount of utilities.  Once you undermine the end of the house you can begin to chip away at the slab and begin to remove the dirt underneath.  As you work your way under the house you will need to temporarily reshore it as you go making sure to keep the house above stable until the new foundation can be repoured.  Modular houses are well suited for this type of project because they are made to be minimally supported during transport and can be shored up several feet inside the outer support walls allowing room to work on your new walls.

In either scenario you will have to make provisions for waterproofing the exterior foundation walls and installing footing drains to keep water from building up along the exterior of the new walls so your excavations will have to extend beyond the perimeter of the exisiting home far enough to make this happen.  A minimum of two feet, preferably three feet will be needed to get down inside the trench and coat the wall with the necessary mastic, put down drain pipe and rock and then backfill.  I have a couple of simple illustrations I am attaching along with this email.

As far as costs go?  Having a house mover lift and shore the house will cost anywhere from 5-10,000 dollars depending on the size of the house and the availability of the contractor.  Some house movers even offer foundation services and work with concrete or masonry contractors to give you a complete package.  The foundation by itself contracted separately should run somewhere around a hundred dollars a lineal foot, maybe more maybe less but in today's market prices are a little better than they were a few years back you might be able to find a hungry contractor willing to give you a decent deal.

Of course there will be plumbing and electrical work involved and depending on the elevation of your external sewer main and depending on whether you want to put a bathroom in the basement you will need a pump for any sewage installed below the sewer main and this may cost several thousand dollars extra.  Then you will have to finish the whole thing off, backfill and replace the sod around the house, drywall and add flooring to the basement, install some sort of heating and cooling down there or upsize your existing furnace and air conditioning.  How much of any of this you can do yourself depends on your abilities and experience.  Oh and don't forget that you will need to get a permit for the whole thing and undergo city, county, village, or state inspections.

Well I hope this information helps.  Please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com

Bruce E. Johnson

Expertise

I can answer any construction related question in regards to carpentry, concrete, drywall, masonry, structural elements of any type of building, residential or commercial. Interior or exterior.

Experience

Custom Commercial and residential buildings. Churches, theaters, schools and auditoriums. Most recently I am working with the Catholic Church on several design build committees. I have a website related to scheduling and project supervision. Although my expertise is more related to multimillion dollar commercial, educational and theatrical projects my generous credentials in residential and remodelling construction make me a viable source of information regarding all forms of building questions.

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