Charlie:
It's a complex formula that varies with AHJs, I've just gone through it. Basically a new home has to have them, a remodel doesn't, a rebuild of a new home does have to have them. When I started my negotiations with the AHJ I decided to leave some old walls up qualify as a remodel rather then building a new house, so many builders were doing this that they went to the Board of Supervisors and passed a new ordinance changing the rules defining Remodel vs. Rebuild, I was able to go by the old definitions by arguing that I was in the "pipeline" and should be "grandfathered in" under the old definition, basically I had to leave 50% of the "contiguous" old exterior walls up, I missed the word "contiguous" and had to redesign. I would have never made it under the new ordinance, my customer said that if he had to have sprinklers and huge ugly tanks he'd not do it and move his family and software corporation out of state, California has gone to far with something this stupid. BTW, it's probably costing me as much to avoid the sprinklers as it would have to have put them in.
Additions over 500 square feet trigger sprinklers as well under their ordinance, so check with your local AHJ to see what ordinances they have adopted to deal with it, as you can imagine people are trying to find any workaround the can sending the AHJs into fits and to their city councils and boards of supervisors, if you win you not only avoid the sprinklers you avoid the new toxic green code, so it's a political and moral issue with people.
If a sprinkler goes off inadvertently, and they do all the time, it can cost anywhere from hundreds of thousands to millions, within 24 hours the home has to be stripped of all carpet, sheetrock, and insulation, a remediation company has to start dehumidification machines that run for weeks while scientists come in and continually take samples back to the lab for mold analysis, when clearance is finally given the home has to be rebuilt. Insurance does not cover mold caused by anything but an actual fires, it does not include accidental or inadvertent release, so it could mean loss of the home or bankruptcy for owners. To give you an idea of what's involved I recently did expert on a window leak where
mold started in that stupid styrofoam the window guys sprayed around the windows, a sprinkler going off would cause the same procedures. The occupants of two units had to be relocated, fortunately not all 15 units, all because of a small leak.