If you want to legally complete both matters on the same day you will need to discuss with your conveyancer the best way to draft the contract for your sale so that it links with the terms of the purchase contract.
The new build purchase contract will usually have an anticipated legal completion date in it, along with details of the completion notice procedure. A common notice clause found in a typical new build contract would be a 10 working day notice and is usually triggered by an event (this would be set out specifically in the actual contract), to give an example the property being signed off by building control or the NHBC (National House Building Council) or other new build warranty supplier. If that was the case we would seek a clause in your sale contract to provide for an 8 or 9 working day completion notice (the reason why we stagger the number of days is just in case the new build contract notice came in late in the working day and there wasn’t time to officially serve the related sale notice the same day).
It is wise for the sale contract to contain provisions for an eventual completion date, referred to as long stop. If your buyer is getting a mortgage to fund the purchase, it would be sensible for them to make sure the contract provisions allow them to complete before a mortgage offer expires.
A further point to consider is making the sale contract conditional on the purchase contract, in the very unlikely event you wished to terminate your new build purchase contract (perhaps due to build delays or a material change being made to the property meaning you no longer wished to proceed). It is however important to flag a buyer may be unlikely to agree a termination clause. Your conveyancer will need to carefully consider how the termination rights in your sale contract and purchase contract work alongside each other.