nadograditi

1. (of a building) to build on top of; to add a storey
   Komšije su nadogradile još jedan sprat. — The neighbours added another storey.
2. (of software, hardware) to upgrade
   Moram da nadogradim operativni sistem na laptopu. — I need to upgrade the operating system on my laptop.
   Nadogradio sam računar novom memorijom. — I upgraded my computer with new RAM.
3. (of knowledge, ideas) to build on; to develop further
   Na fakultetu ćeš nadograditi ono što si naučio u školi. — At university you'll build on what you learned at school.

The imperfective counterpart is nadograđivati.