Is this permission necessary under the lease agreement? Was planning permission required separately?
Is this the case for your alterations?
In theory yes. But I know from experience that this is a bit of a grey area (as is the whole issue of management companies in general in many respects). Firstly what's "in keeping" with the area can be very vague in terms of its definition in the management agreement lease and secondly the management company (including you as pointed out above) may only be able to enforce their rules by resorting to law at the (potentially significant) expense of the company/householders. If you are confident that the alterations are not in breach of the (possibly vague) management company lease agreement rules then stand your ground. It's your house.