Hi Tosa
Presuming that the courtyard is on title of your lot, you will need to apply to the owners corporation to make a by-law entitling you to carry out building works pursuant to section 52 of the Strata Schemes Management Act 1996 (on the basis that the strata scheme is in NSW, although, other state legislation will provide for similar arrangements). This provision empowers the Owners Corporation to confer on you certain rights of exclusive use and special privilege with respect to the common property to enable you to carry out works to your lot. The Owners Corporation must make a by-law of the sort in a general meeting by virtue of a special resolution.
Provided that you are not proposing to exclusively occupy a particular area of common property (for instance, building into the air space above the boundaries of your lot) or the proposed construction will not detract from the appearance of the strata scheme, then the Owners Corporation may approve such a by-law. (Notwithstanding this, if you offer suitable compensation, the ulterior position may be more palatable to the Owners Corporation).
Ordinarily, the by-law will require you to provide indemnities in favour of the Owners Corporation in relation to damage to common property (such as external walls etc) and you will be required to provide evidence of local government approval of the works prior to carrying them out. Ordinarily, there will be further terms relating to the conduct of the works (ie. control of noise, transportation of building materials etc).
In terms of an amendment to the strata plan, if the total dimension of your lot is not changing, then, once the works are complete, you may lodge a building alteration plan at the Department of Lands recording the amendments to the boundary of the inhabitable area of your lot (although this is not necessary). There may be some issue with the effect of the alterations on the unit entitlement of your lot (for instance, the Owners Corporation may consider that the value of your lot has risen thereby creating a disparity with respect to the levies which you ought to pay).
The foregoing issue, however, digresses from your initial query. Should you require advice concerning this matter or you would like assistance with the drafting of an appropriate by-law, we would be happy to provide our fee estimate.
Best regards
Loraine Booth
Teys Lawyers