# CA Hillside Addition Feedback Please



## Calisota (Apr 15, 2011)

I have a client that will be under a design-build contract and am about to get my head into their project. Primary scope of work is a full gut renovation of all interiors. Secondary scope of work is a feasibility estimate for a modest addition to the home.

I'm doing some preliminary research before getting into it with an engineer. I'm looking for a new one, as the last addition I had was back in 2008 and that old gentleman has moved on. Until then, I thought this might be an interesting chance to share and get feedback on CT. Hmmmmm . . . . what was it about 2008 again? 

This is a typical CA hillside overlay district and subject to mandatory soils and engineering for any new construction, which boils down to foundation design and cost increase. No Type V construction for hillside work. 

I have not had the opportunity to do a hillside addition yet in CA, but know what to expect and the ordinances governing hillside development. I live on a hillside and have had 10 years to get to know many of the requirements of working on slopes out here. What I am handicapped by is direct experience in foundation work on slopes.

My current understanding, from a feasibility standpoint, when approaching an addition on a slope, the greatest attention is typically on the "downslope" portion of a foundation, usually resulting in substantial Caisson placement and spacing per engineer and soil report. Associated costs are dictated by equipment access, size, depth, and the number of caissons. "Upslope" portions of the foundations typically do not have the same design elements when it comes to calling for caissons in the design. Am I off base anyone?

The dwelling sits lot line to lot line basically, no heavy machinery access to the rear / up-slope part of the property. Up-slope rear of dwelling is level grade for approximately 12' +/-, full width of lot. Ample space for Addition design. Down-slope in front of dwelling is sloping grade. 

So, which design direction would you explore 1st? Up-slope addition behind the dwelling, or Down-slope addition over the garage? Upslope may be more economical foundations, downslope may provide better access from garage to interior, but possibly cost prohibitive. I've got a couple images to reference from below to get the ball rolling.

This is a new experience for me both on here and in the real world. I'm looking forward to seeing how it all turns out.

Thanks,

Joel


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## Chris Johnson (Apr 19, 2007)

Until you have the soils report and an engineer to review you cant guess at any of it.

I have done many a hillside foundations where it was a typical footing, well sorta typical, if the wall was acting as a retaining wall I've had footings 6'3" wide and 1' thick and some 1'6" x 1'6" with a 22' high concrete wall on it.

I've also had it where they drove 16" caissons 20+' into the ground and then a grade beam/footing prior to a concrete wall.

Every job was a crap shot until the engineer reviewed and designed.


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## ScipioAfricanus (Sep 13, 2008)

HI Joel, off hand it looks and seems to me that building up and out from the garage would be the thing I would look at first. 
The garage and retaining wall footings and foundations may need to be underpinned at some point but when done will allow great access for construction of an upper story to attach to front of the house.
It would also provide some possibly much needed design opportunity for the front facade.

Andy.

BTW by "Caissons" do you mean pilings?


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## Calisota (Apr 15, 2011)

ScipioAfricanus said:


> BTW by "Caissons" do you mean pilings?


Hi Andy, thanks for stopping by. I'm no foundation expert, but my understanding are that caissons are poured in place and pilings are driven or inserted.


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## ScipioAfricanus (Sep 13, 2008)

Ah, I see, makes sense.

I am just more familiar with the word caisson being used in bridge building and dock works. 

Andy.


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## Rio (Oct 13, 2009)

If you decide to go over the existing garage it's likely that the retaining walls for the garage walls will have to be beefed up in some way. 

If there are existing plans on file and it can be shown that the new addition won't overload them it might be a good way to go but if not there will have to be some rework done. This might turn into a can of worms for a number of reasons.


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## Calisota (Apr 15, 2011)

I just checked title and I was wrong about the building date. Actually 1939 by looks of it. I would have little faith in an engineer qualifying the existing garage walls/retaining walls supporting a new second story. Also, the garage is a 1.5 style from a bygone era. The clients are mainly interested in a front addition if there is a way to build in an interior entry to the home, ie setback second story off the garage. The rear desire is motivated by larger master and better family flow to the rear terraces.

The Can of Worms Rio alludes to, is in my mind, inevitably tearing out the existing garage walls, widening to a true 2 car width, as well as including interior stairwell access to the home. Looks grim from a ROI point of view to me. New slope cut back, new retaining walls, THEN getting to the addition value would be a steep ticket price my gut tells me. 

The square footage apples to apples between front and rear possibilities, is not equal either. The rear potentially holds a 70 to 100 sqft advantage overall. I believe that I can get around 350+ SqFt in the rear. For urban environment, that's good square feet. The front, with setback requirement, may only yield high 200's.

Challenging to find a valuable solution to say the least. Neighborhood can handle the extra square footage for the right price.


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