You can always offer below asking, but as Karen and Ed noted there is usually a small margin for profit in new construction and you might not be successful in your first offer with a deep discount. Of course, if you don't ask...
If my buyer asked me to write an offer with a deep discount on new construction I would ask them to answer a couple of questions for themselves first. One, I would ask how badly the buyer wanted the home; are they willing to risk loosing it? Two, I would ask them what other properties they had seen and why they were choosing to offer on this builder property. Sometimes evaluating these questions will help a buyer determine if a deeply discounted offer will ultimately accomplish their goal.
Good luck, and let me know if I can be of any further assistance.