Our company works with a lot of remodels and new construction. Our answer is "it depends".
1. It depends on the level of amenities you want in your new bedroom and bath (tile type, sinks,
faucets, shower, etc)
2. It depends on the current construct of your home
A. The location of plumbing and electrical and what needs to be done to existing
utilities to add on.
B. What needs to be done to add to your existing roof configuration
C. What engineering work needs to be done to add to the existing floor configuration (slab,
crawl space. etc)
3. It depends on the level of plans needed by the city to attain the permits
Our advice is to contact 3 general contractors in your area, sit down with them and discuss the items above plus what ever else they say is needed to have it be up to code. You will want to get clear on flooring, types of tile, types of sinks, faucets, shower floor, tile and heads, lighting fixtures, etc. All of these items effect cost per square foot.
As for your tax bill, when you are finished you want to contact the County Tax Assessors office to get the new square footage of the home on the tax record. Ask in advance how it will effect your tax basis. You want the new square footage on the record for when you go to sell the home. Nothing worse than your home being 1,350 square feet and tax record showing 926. An appraiser usually goes off the tax record and the home would then appraise of the 926 sf size.
We hope this helps, let us know if you have any further questions.
Mark & Kari Shea
Shea Real Estate
National Association of Realtors
CA BRE License 01713506