Do i need to provide entire app project code for additional feature to be built?

Hi gurus, I have a general question. I'm thinking of using a certain developer to build my app, however he can not build a particular function. But he knows how to build the rest. So I'm thinking of getting him to build the app without the function he can't build, then getting another developer to build it at a later stage. I rather not give the second developer my entire code for the app, since it's only a small function they need to develop. Is it possible only to provide a section of the code, or will they need the entire project code?

Thanks in advance. Your help is appreciated.