Hi Hal,
The decision of building the pool during construction or post construction was a decision that kept me up at night. It wasn't easy. I wanted to build after construction to be sure that the pool will be built at the correct level; i.e same height as the outdoor room concrete slab. However there is a clause in Metricon's 30 year guarantee document that mentions that the guarantee is void if building a pool afterwards. As my pool was within 1.5m of the house I didn't want to take this risk. Due to the height of my house and the accessibility for excavators and cranes it would of been difficult building the pool after the house. Therefore I decided to let Metricon know that I'll build the pool after they cut the site. Doing this also involves a bit of a headache because there is a handover process from Metricon to me and then back to Metricon after the pool has been done. As the pre-construction process was very slow I consulted my customer support consulted on if I should just go ahead and build even before Metricon do any site works. She told me just to go ahead which I did. It was a good decision because the pool was built just in time for Metricon to do their thing. The level of the pool was within 5cms of the outdoor room concrete slab which was actually planned. If you build the pool afterwards then make sure you let the builder know in advance so they can run the sewer pipes so it won't obstruct your pool.
You need to tell builder that you are building a pool and they will work out if piering is required. If the depth of the pool is greater than the distance from the slab then piering is required. In my case it was so so I paid an additional amount for piering at the rear of my outdoor room slab. From memory it was roughly 3K. See image below showing the engineering for the outdoor room slab. It has 4 bored piers.
Metricon requires that the pool has a OHS complaint pool cover which is engineered. Expect to pay 3K+ for this.
If building a pool prior to home construction I would highly recommend that you wait till you get the final plans for the house which show you the finished floor levels for your house. The pool company will use this to work out what elevation the pool should be installed at. The levels are measured by referencing the TBM(To benchmark) which is a nail inthe kerb that the builder installs during the site survey.
I hope that helps.