My question is whether this loan amount is even manageable? According to the financier, yes, but better to check yourself than just trust.
Unfortunately, no one can tell you that. From the banks’ side (which usually apply flat rates), it will certainly work. Whether it will work for you, only you can know. Therefore, you should ask yourselves:
What expenses do you have? Also those that do not occur every month, but only once a year. It is best to keep a household budget for a certain period of time so you know where your money goes. Also calculate with the expenses that will come when the third child arrives, when you need childcare for 3 children, etc. pp. Think also of additional insurances, reserves for the house (as already mentioned), a new car (if you depend on it, sooner or later a new one will come)...
How much do you save per month?
How secure are the incomes? Is the salary increase guaranteed?.....
You don’t have to answer these questions to us, but to yourselves.
Accordingly, no one can say whether that is too risky or not.
Personally, I find €310,000 quite a substantial figure with a sustainable income of €3,800 at the moment, but that is just my personal opinion and doesn’t mean you couldn’t manage it.
In my opinion, you should first look at how much you can actually afford, not just on paper, but based on your life circumstances. Maybe also consider consulting an independent financial advisor.
Only then, if in doubt, should you look at what things are really important to you and what you might be able to do without. Simply saying now: "Then I’ll just do without the basement," isn’t necessarily the most constructive suggestion. Because you probably had a reason to want to build with a basement, right?