I think the bigger question is when you say "available large parts of the day" do they want you on site at their location during that time or simply available v

I think the bigger question is when you say "available large parts of the day" do they want you on site at their location during that time or simply available via phone/email during that time? If it simply phone/email, you could consider like @hoodoof said with setting up a tiered support pricing plan. With phone/email only you can still work on other projects you just need to respond to their requests promptly during that time which can be worth an additional amount. On the flip side if they want you onsite during that time it becomes a completely new ball game as you are unable to service other clients during that time which can impact your income. In this case I would build that cost into the contract, not necessarily charge a retainer. Finally, as @y0mbo was saying, if the client just wants to know you are available during their office hours compared to working at night then no you can't charge an additional fee for this. This may be the case if they are in a different time zone and want to ensure that they can reach you to discuss the project and such. Again, if this is the case I would build the cost into the contract.