I m travelling to the US in June on the VWP and have a return flight 10 days later but I may want to extend this but still well within the 90 day limit. what are the implications on re entering later in the year if I actually extend once im over there ??
Travel plans are subject to change, and the CBP know this. Indeed one can even apply for an ESTA with provisional, if not indefinite, plans. If admitted for entry in the US under the Visa Waiver Program, you'll be authorised to stay for a maximum of 90 days. The important thing is not overstaying, rather than extending a provisional travel plan.
When the immigration officer asks you about your plans be honest and you should be fine. Mention that you thought of staying X days when you applied for the ESTA, but are now thinking of staying more than that for whatever reason. As long as the reason is valid for admission under the VWP you should have no problems. Should you be question upon re-visiting the US, the advice is always the same: be honest.
For more information on the VWP and ESTA, see our canonical question on the topic.