East Mount Road is already in a conservation area, but the article 4(2) directive gives the local planning authority further powers over changes to the elevations of single family dwelling houses fronting a relevant location (which in this context means a highway or open space).
You can download this directive on the right hand side of this page.
Plans Processing Unit
City of York Council, 9 St Leonard's Place, York YO1 7ET
tel: (01904) 551553
email:
planning.enquiries
@york.gov.uk