Basically you need a special resolution by-law that allows the deck builder to use such common property as required (like bits of wall and ground it is attached to) in exchange for their agreement to repair and maintain the common property at their own expense, now and in the future. Normally the person installing the deck would pay for the legals (since they are the only ones benefitting) although the OC might want to choose the lawyer. if the installer isn't prepared to do this, you'd be nuts to let them go ahead as responsibility for the deck would fall back on the OC when the owner sold. If the installation of the deck leads to substantial increase in the value of the lot, the OC is entitled to some remuneration calculated on the basis of increased value minus cost of installation (including legals and paperwork).