Acknowledging your limitations can build confidence.
I've been there. I know the feeling. A customer asks you if you know how to do something beyond the service you are capable of providing. Panic sets in as you try to formulate a response. There are two ways you can deal with this situation. You can let your panic guide you and say, "Yes, we do everything." and then worry later about how to back up your statement, or you can be open with your customer.
How you deal with this issue has as much to do with your customer as it does with you. Of course you want to win the business. Of course you want your customer to be impressed with you and your firm. Of course you don't want to leave money on the table. But look at it from the customer's perspective.
Perhaps your customer does not understand the process or technical complexities of website creation. They may not realize that there is a significant difference between designing a website and designing a billboard. They may not understand that copywriting, HTML coding, search engine optimizing, layout design, image optimization, and all the other specialized skillsets that go into the creation of a successful website are unique and are commonly exclusive of other skills. If this is why they have asked you to do something beyond your capabilities, you will do your customer a service by educating them.
Perhaps your customer doesn't really care who does the work as long as it gets done. Where the ignorance described in the first example can be corrected with knowledge, this example is based on apathy. If the customer truly does not care about the quality of the final product, they'll probably let you stumble through the unknown, learning as you go. But even if you get some work out of them, it won't be work you're proud of - work you can show future prospects that will help you grow your business. You will never impress this customer, and therefore, they will never develop an affinity for you and your work. This customer is not worth having.
Here's another perhaps, but one that is far more positive. Perhaps your customer is pleased with the service you have provided so far, and they want to see how they can make more use of you and your firm. They may know that their request is outside the realm of what you have done in the past. They may want to see if you offer more services. Because you have earned their trust and respect, they may want your assistance solving larger problems. This is the sort of customer that you can build a business around.
Now - back to you and how you respond. If the first customer simply needs to be educated to become a great customer, how does it help your business if you keep them in the dark? If the second customer never sees the value of professional work, how will they ever have a lasting positive effect on your business? Since the third customer has already decided they want to be a long-term positive influence on your business, why would you be anything but honest with them?
The plain and simple truth is this: no one expects you to know everything. No one does. As your opportunities grow, team with other experts that are as good at what they do as you are. You won't deposit every dollar offered by your customers, but your customers will be grateful for a long time.