The big variables are where you live, the design, how much work you will do yourself and the materials you select. If you live in a rural - low cost area the price for your addition will be less than 1/2 of what you would pay in Manhattan, etc. Materails - same thing you can pay $30 for a faucet or $300???? And there are a lot of these material decisions in a bathroom. The 2nd story adds another variable especially in CA where the first story needs to hold up the 2nd in case of an earthquake so the existing structure may need a lot of reinforcement.
Having said all that there is no reason why you should not get a ballpark and that ballpark is $250 a square foot...plus or minus $100 a square foot (sorry for such a big range) I got that from the website RemodelOrMove.com which has a free to use online calculator for people who just want a ballpark estimate of remodeling costs. I pasted the numbers i got from the calculator below.
Estimated Report for whole house calculator...
After careful review of your project where you Hire a contractor, do None of the work yourself and use Average cost materials, we estimate the cost will be between $89,900 and $104,000. We also estimate the payback (the amount your home will appreciate by doing the remodel) will be $45,600.
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Do you need more detailed information about the calculators recommendation or your situation? Click here to see the full report or just email us your question and we will see if we can help. Answers@remodelormove.com
If allowed, it is likely less expensive to demolish your garage and slab and build a brand new two story structure with garage below and living space above. This will cost you a minimum of about $150/SF including plans and permit fees. I say minimum because I don't know what kind of home you currently have and what level of construction you want. I(t could be a lot higher.
To build upon an existing garage will require you to enlarge footings below the garage wall, to add Earthquake reinforcing to the garage walls and to connect all this to your new structure above. Also you will have to remove the old garage roof, so in the end all you are saving are the four garage walls.
The city does not really like to have converted garages, but you can add a rec-room to a garage.. there are all kinds of requirements... like you can only do a 1/2 a bathroom and not a full bathroom.. and many more...
Additionally setbacks are a concern.. any new construction would have to meet today's setbacks from the edge of the lot... Is this a detached garage? if so, it cannot be within 10 ft of the current home...
Additionally if you are doing a 2nd floor you need to change the footing of the the building and some framing... so make sure the load on top of the building will be supported... you will need an engineer to stamp the plans.
I am a general Contract or and a Real Estate Broker... so I have some idea about what I am suggesting... is not a s easy as it appears.
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Best of luck!
I have never gotten an estimate for any contractor without making an appointment to have them come out and see exactly what I wanted done, take measurements and come back with an estimate.
I can tell you this much for sure, it's always more expensive then you thought it would be!
However it may be extra if the existing footings/foundation require enhancing to support the extra load. I have good designers, etc. Tony 818 445-6918.