Starbuck

Hi, I hope you can help me, I have some questions about the Thai tourist visa. My partner and I are going to Thailand (from the UK) on the 17th of April for tourism purposes. We believe we need to apply for the 60 day tourist visa, however we are hoping to seek out the 30 day tourist visa extension when we arrive and stay for 89 days total. We have booked a return flight for the 89 day stay under the assumption we will have the extension granted. So my question is, when we apply for the tourist visa do we need to put our intended date to leave within the 60 day visa for it to be granted? And if so, do we need to book an onward flight within the 60 days? We are also unsure what to state on the declaration form for when we enter the country, as that asks for onward flight details. We are concerned that we will get into trouble if we put our intended day to leave within 60 days, but do not have a flight booked within that time.