L ONDON — The end of summer is a nervous time for superyacht designers, and not because they fear that the owners of their latest creations may be disappointed with the first outings in the Mediterranean.

The worry is about the designers’ next vessels, because this is the time of year when clients whose boats are still in production come back from holidays with a wish list of new features — usually, based on what they saw on their friends’ yachts or at the Monaco Yacht Show, which ended Sept. 28.

“Right now we are quite far down the line in completing a big yacht in northern Europe for one client who has just spent time on a friend’s boat, which is not necessarily helpful,” said Dickie Bannenberg, the head of one of the world’s best-known superyacht design houses, Bannenberg & Rowell. He was in his London studio, an airy two-story space lined with sleek models of its creations.

“The delivery date is in the first half of next year, and that is sooner than it might seem,” Mr. Bannenberg said. “It’s fine when it’s superficial — let’s say they liked the plates or towels on their friend’s yacht — but if you’re not careful it can verge on, ‘Oh, my friend’s gym was like this, can we have something similar?’ or, ‘I would really like to add a submersible vessel.’”