I plan on installing LED lights under my dry-below deck using 14-2 UF Romex. Can I connect the UF wire to an existing exterior light (NM Romex) that is switched from inside the house? If not, do I have to run the UF back to the panel and install a separate switch to control the new LED lights? Switching the new lights from the existing switch would be ideal.
Yes, in fact, UF wire can be treated just like NM wire when ran indoors.
340.10 Uses Permitted
4. Installed as nonmetallic-sheathed cable. Where so installed, the installation and conductor requirements shall comply with Parts II and III of Article 334 and shall be of the multiconductor type. Including the ampacity adjustment in 334.80.