When I started my firm, I didn't have much experience with business. In particular I felt unsure about money matters. Luckily I stumbled upon a business attitude that partly made up for my lack of experience.
I discovered how to apply certitude (real or feigned) to replace uncertainty.
Money matters require experience to reach a level of comfort and confidence. Business courses can help, but when it comes to asking to get paid for your work, there is an enduring level of discomfort. The reality is that your clients are expecting a business-like approach from you. So you should give them one.
For instance, even though it may be the first time that you are asking for an initial payment, if you say that it is your office policy, you will rarely get any push-back.
Whether you ask for an initial payment is up to you. The point is that your clients will take your word for it that it is standard practice (and if you say it is, then it is). The vast majority of clients are embarking on a construction project for the first time. They expect you to show them the way. They assume they have selected the right firm. They expect you to know what you are doing.
It is tempting to share your uncertainty, to be open about your uncertainty; but you may scare the hell out of your client instead. Be the rock they hope they have hired.
There are good reasons for initial payments, reimbursable expenses, limits on Schematic Design time and on Construction Administration time. If you feel that these things are good ideas, you simply make them your policy without apology.
"They will take your word for it."
Some articles that touch on related ideas are: