From NFSA TechNotes April 11, 2017
Is it permitted to use brass extension fittings as part of a drop to extend the sprinkler deflector below a lay in ceiling in accordance with NFPA 13-2010?
Answer: While NFPA 13-2010 does not include specific criteria regarding the use of extension fittings, the answer is “yes, if the requirements of NFPA 13-2016 can be used.” This would require use of the equivalency provision of NFPA 13-2010, section 1.5.
New language was added to the 2016 edition of NFPA 13 to provide requirements for the use of extension fittings. NFPA 13-2016, section 6.4.8 allows the installation of a single extension fitting up to 2 inches without inclusion in the hydraulic calculations, provided that the internal diameter of the extension fitting has the same nominal inlet diameter as the attached sprinkler. This requirement would be applicable for extension fittings used with k-8.0 and smaller sprinklers in light and ordinary hazard occupancies only. Installation of multiple (two or more) extension fittings would not be permitted for larger dimensional needs. If needed, extension fittings having lengths greater than 2 inches could be used if specifically listed. However, these extension fittings would be required to be included in the hydraulic calculations.