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:
  • 50% or more of the gross floor area of the existing building is demolished. This excludes basements. The gross floor area in this one and only case is measured to the inside of exterior walls; it includes stairs, interior walls, and everything else inside.
  • 50% or more of the exterior walls are demolished (not including foundation). Oddly unlike the floor area provision, this is measured from corner to corner on the exterior, and includes any part of the wall such as top plate or sheathing.
However, Montgomery County does NOT require sprinklers for additions if the house is not sprinklered - regardless of size (unless, of course, in building that addition you are also doing 50% demolition of either floor area or exterior wall).

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).