If you are planning a custom house, you will also receive an individual cost estimate that exactly includes what you need/want.
I am not sure whether you want to stick as closely as possible to the standard offer to save costs or if it is more important to you to get a plan that exactly meets your needs while making the best possible use of your building envelope. Of course, it is also a question of budget.
The standard houses are usually cheaper if you take them exactly as offered. Any change becomes relatively expensive.
A custom-designed house is naturally more expensive at the beginning, but then you pay for what you want and don’t have to negotiate every change.
I find that understandable, because with the standard houses, the builders of course save on planning costs. However, it must be noted that the standard equipment of such houses is usually kept rather simple and cost-effective. Then you can state a nice final price as “turnkey” and attract customers that way.
But as I said, I am not so sure which direction you want to go. Is free planning an option for you?
And that builder, is he already set in stone? Especially when you have to plan a bit tricky, it is worth considering thinking about timber frame construction, as Yvonne has already mentioned. Much more is possible there. Or is that not an option for you at all?
We knew we wanted a wooden house but visited several providers and discussed our floor plan with all of them to hear how they deal with it, what works, what doesn’t, etc. That was quite interesting (one saw no static problems with our large open room, another wanted to install a support pillar there, etc.).
In the end, we ended up with a small craft business and will plan our house individually. We have a similar problem to you, namely a very restrictive building envelope (our house will be 8.40*12) and just wanted to realize quite a few things.
We also visited Baufritz, for example, which offers very high-quality wooden houses. With them, you can both plan individually and have standard houses off the shelf. The standard houses are very attractive in terms of price, but every little change really hits the cost hard. Individually planned houses would then be well above what we now have as a financial framework.