Quantity Surveyor for home construction

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Am about to embark on a new erection of an inter-terrace, and was wondering what the scope of the Quantity Surveyor should be.

Am told that I can either engage the QS to do up the contract (ie only involved at the beginning of project), or engage on a full scope basis to also take care of cost assessment etc for the entire life cycle of the project. Difference in cost is about $7,000, so not that little but also not significant when considering the anticipated construction cost.

For those who have gone through a similar process, is it necessary to engage a QS on a full scope basis? Or just take care of front end will do?