Builder is looking for example 300k for a new build. Can people bid on these properties?
Two three bed semis for example, side by side, both buying off the plans. Could one person potentially pay more for the property?
Not entirely sure what exactly you're asking. If the builder finds that the first of the two houses sells off the plans easily, s/he might decide to ask €310,000 or more for the second.
Or are you asking if you can haggle with a builder over the price? Yes, although in the current market I doubt that you'd get very far.
Either way, until contracts have been exchanged, the price of any property is open to negotiation, up or down.