It depends what your Owners Corporation rules say. If you have default rule 4, then erections and alterations require a general meeting resolution. However, default rule 4 is one of the rules that an OC may amend by special resolution.
So, it is possible that you have an amended rule for Erections and Alterations that allows the Executive Committee to approve some or all sorts of alterations, with or without guidelines from general meeting resolutions that constrain the EC in what it can approve. Common constraints on the EC that an OC might have imposed include only being able to approve a unit alteration if materials and colours remain consistent with the rest of the scheme or subject to consultation with neighbours or if certain architectural features are preserved.