The International Residential Code requires an automatic sprinkler system in all single-family and two-family dwellings (par R313.2 in IRC 2018). The IRC has an exception: sprinklers are NOT required for additions or alterations to existing buildings that are not already sprinklered. But...
Montgomery County added their own provision (in Regulation 4-15AMII daed 8/3/2015) adding paragraph R313.4 requiring that sprinklers be installed in existing houses when either:
For new houses, the county has required sprinklers since 1/14/2004. If by some fluke your house was built after that without sprinklers, you will have to add them when you renovate or expand. Residential sprinklers are designed based on NFPA 13D, which is significantly simpler than NFPA 13R (multifamily) and NFPA 13 (taller multifamily and commercial).