Mini – I don’t think matters are as complex as all that.
It’s increasingly common these days for the original dwelling on a parcel of land to be retained whenever the site is subject to a multi-unit development; this usually (but not exclusively) occurs when the dwelling is heritage listed.
When the completed development is covered by a strata title, the original dwelling, often refurbished to the extent permitted by its Listing together with a defined area around as “garden space”, is given a lot number and a unit entitlement in the overall Plan.
In your example the By-Laws grant “exclusive use and enjoyment” and as that type of By-Law usually applies to areas of the Common Property that can be enjoyed, I doubt that the intention is segregate the responsibility for the overall maintenance and repair of the buildings themselves. As other posts have alluded to, this would give rise to the complexity of differential levies, and as I’ve said, I don’t think that’s the intention.
So no Mini, I don’t think that the Owner of the house would be individually responsible for external maintenance and repairs to that house or that they could make alterations such as those you mention (e.g. solar panels, water tank etc) without Owners Corporation (O/C) consent.
Rather, it’s more likely that Proprietors’ Contributions to the Plan’s Administrative and Sinking Funds and the expenditures from those would function in the normal way across all the Common Property, except those covered by the Exclusive Use By-Laws – where the Owner of the house wouldn’t contribute to the costs of overall grounds maintenance (i.e. gardening and grass cutting around the building), and the Owners of the other Lots wouldn’t contribute to those activities on the “garden space” of the house.
I hope I haven’t made this (even) more confusing Mini, but if you’re serious about purchasing the house, ask your Solicitor / Conveyancer to arrange for a Strata Search, and if the Plan is operating (i.e. a functioning O/C) specifically request commentary on the operation of the Exclusive Use By-Laws, and if it’s not operating, request a Legal Opinion on how it would operate.