I don't think PP needs to cost much if you already have drawings, it's more the time it takes if you're in a hurry so best put the plans in soon if you want to build later this year. You can do them yourself (but have to use more than just a few crayons ). Jenny did ours on graph paper using a ruler and pencil. But if you're adding 2 rooms your builder should get some drawings anyway.
You'll have to pay something but not much in the grand scheme of things for a buildings inspector to check the build as it progresses so you get a certificate at the end. The cost is on a rising scale according to the cost of the work to be done.
The inspector will check your foundations are deep enough, your builders use proper fireproof materials and that sort of thing - he's working in the interests of you and future buyers of the house and anyone else the house can affect.
When we removed an outside toilet and replaced it with a drain cover, he insisted on checking the drains and it turned out one had collapsed so we had to fix it even though we hadn't done anything to affect it. Cost a load to dig up the path at the side of the house and then he insisted we disconnecte the power cable out to the shed because it wasn't in proper conduit or something like that.