Our ranch-style house in Yuba City, California, has a guest wing with two 10'x12' bedrooms divided by a bathroom. If we bump out the gable end of the house four feet for room additions we'll end up with a bigger bathroom and bedrooms. The bathtub and toilet would be moved out four feet and a double-sink put in the bathroom. Each bedroom would gain a 10' wide closet and the old closets would be opened onto the bedrooms. The lot is level, we're on city sewer, the simple roof is asphalt shingles, there'd be no new windows. Are we talking $50,000? $70,000? More?
Manuel O. ~ Yuba City, California
Hi Manuel, It sounds like a very simple addition; it doesn't get much easier than adding on to the end of a one story ranch home. The only difficult part might be moving the toilet and tub and achieving proper fall for the drain lines. It could involve doing a little demo of your existing floor.
One thing I'll mention before getting into costs is that you might want to think about making the addition a little larger if you have room on the lot. From the way you describe it the addition sounds like it would be about 26 feet by 4 feet for 104 square feet. I arrived at the 26 feet due to each bedroom being 10 feet wide and most hall bathrooms are about 6 feet wide. I know that sounds like exactly what you need; making each bedroom a little larger and taking advantage of the addition to make the bathroom a little bigger, but if you're going to all the trouble of bringing all the contractors in to do the work, you might want to add a few more square feet while you're at it.
I'm way over on the eastern side of the country and am not sure how construction and contracting costs run around Yuba City, California, but I would think that you could easily do this addition for around $150 per square foot. That would put you at about $15,600 for the addition and I couldn't imagine it going over $20,000. You have no windows or exterior doors, the roof trusses should be simple and the framing should be easy. I imagine you'll use the same bathroom and bedroom entry doors, but you might need new closet doors. You'll have some insulation and sheetrock and probably want to install new flooring in the bathroom and bedrooms. Your electric work shouldn't amount to much and with an addition that small the size of your existing HVAC system should remain adequate, but you might want to verify that.
You don't mention what your exterior finish is, but even if it's brick or stucco, that little bit of exterior wall area shouldn't cost very much. Since the addition is at the end of the house you shouldn't have to change your driveway or any of your landscaping at the front of the home; all in all it should be a very cut and dry home addition.
I recommend that you have at least 3 reputable contractors take a look at the project and give you estimates. These contractors and their people are going to be working in your home for a while so make sure you check references and that they have contractor's licenses and insurance.