The certificate of title of the strata , not the lot contains all the information you need.
The various lines drawn can be confusing.
The strata plan will have a table of all the area of the lots. The areas include the area of the residence, the garage and any courtyards that form part of the lot.
The prolongation you mention generally define the boundary between lots or between a lot and common property. These lines usually are where there is no building to define the boundary.
More modern strata plans will have a vigiale (a little curly sign) between the internal lot property and external lot property. It indicates that the lot property continues into the other area.
.If the owner has encroached on common property by enclosing it, issue them with a notice to comply for breaching the common property by law. Alternatively have an exclusive use bylaw drawn up. Since this increases the value of the owners lot, make sure there is some compensation to the OC is paid (yearly rent).
If the enclosed area is common property, then the OC can enter this space at any time in order to do repairs. The lot owner is also in for a surprise, since this is common property, the OC is not responsible for restoring any damage to items the owner put in when effecting repairs.
You can get a strata plan from one of the LPI agents online, but you should have a copy attached to your contract of sale. The strata plan almost never changes so the one you have is good.