whether and how one can negotiate now depends on many factors. before signing, there is ALWAYS more room than after signing.
1. with banks you can compare online, but the differences are often not that big. hardly anyone really stands out. once you have compared a few banks, you quickly get a feeling for what is good and what is not. the online calculators are not bad at all, as long as they explain their stated interest rates along with the conditions. there are basically no interest surcharges if the loan is shaky. then there is simply no loan. if you have real equity, you should make sure to reach at least 80% loan-to-value. we now have 82%, but the interest rate is so low that it was probably calculated with 80%. with banks, there is not much room for negotiation after the comparison, because all already calculate low to attract customers. where there is still something to talk about is a longer interest-free commitment period (because it's nonsense anyway) and general conditions like special repayments (are rarely made anyway, the banks know that of course) or free rate changes (often changing the rate 3 times, we have 2% repayment and can change 3 times between 1%-3% repayment).
2. with construction companies you can negotiate a bit BEFORE signing. depending on the company's principle, there is more or less room, and depending on the order situation, there is more or less room. companies that advertise that something is included for free in some package from certain dates are generally always more negotiable than companies that always quote a fixed price. the former try the discount trick, although that is of course nonsense. (the free extra is of course also included for free on request outside the promotion period...). in the end, you always pay about the same amount for a certain service. if someone is significantly cheaper, they also deliver less or worse. good motivated employees with high expertise in craftsmanship always cost 2-3 euros (wage) per hour more than the bad ones, who also constantly change and the company loses knowledge. whether a company is good can be seen, among other things, by how they pay their employees. it's best if you know someone. very good have also been future neighbors. they can already report a lot about their contracted company and share the stories of the neighboring houses. price-wise, about 5% was possible without much negotiation for us. his house from the catalog was supposed to cost sum X. we asked for 2 extras (roller shutters, fireplace) and asked him for an offer. on a piece of paper he wrote down the sums, drew a line and added 5% lower on. that was his price, no haggling, etc., and done.
3. with direct assignment of subcontractors/trades, there is certainly the most room, because each company "fights" for the order.