The builders solicitors works for the builders not you, don't listen to a word they say! In fact you shouldn't even be talking to them.
You're paying your own solicitor good money to act on your behalf and with your best interests in mind. He also has a duty to do his job correctly which involves satisfying the mortgage lenders conditions.
Complying with planning is a major issue. You have no way of knowing that it will be resolved within 3 years, but it might only take 3 weeks.
However as far as I recall the solicitor can't actually stop you signing anything, you can insist on signing the documents but the solicitor will almost certainly ask you to sign a disclaimer saying he advised you not to. He would also have to inform the mortage lender if any mortgage conditions have not been complied with i.e. planning. They may or may not withdraw the mortgage offer, but if they do then you can't complete the sale and may lose your deposit and leave youself open to the possibility of being sued by the builders for completion of the sale.
Buying property is always stressful, especially when all the strings seem to be pulled by other people, so sit back, relax and let YOUR solicitor do their job. Easier said than done though ;-)