Clients asking for admin panel

Hi,
I'm doing my first steps in getting into freelance web design, I design and code everything myself without using any CMS.
The problem is when a client asks for admin area to add or modify content, usually showing off some new products. I don't know if it is possible to setup a CMS on a HTML website without having to change the whole code and keeping the design?
What do you do when facing this situation? Do you code your own admin panel or use CMS?
Thanks in advance.

See original post by elyas1325