How do you use Source SDK to make Zombie Panic Source maps?

I already have some Source games like Portal and Team Fortress 2 so I can already download Source SDK and I can make playable maps for TF2, but now I want to make maps for Zombie Panic Source and I don't know how to make ZPS maps since it's a HL2 Mod and I don't own HL2. So my question is: Do I need to buy HL2 or HL2:Ep2 in order to make a ZPS map using SSDK?