My simple answer to this question is as soon as possible! Too often a Quantity Surveyor is only brought in when there is a crisis. If a QS is engaged early, then the crisis may be avoided altogether.
Engaging with a QS early to understand how a QS can help is vital. It doesn’t necessarily mean any commitment at that stage but it is better to ask for advice earlier rather than find it is too late.
Often people try to do their own thing without help and this is the hard way. I guarantee I can find you the right contractor quickly and efficiently without YOU having to worry about doing it correctly.
Doing things on your own can mean taking the wrong route, unsuitable for your situation.
Why not call a QS, worry less, sleep well and be content that your project is in good hands.