Planners have ordered landlords to tear down eyesore extensions on 24 homes in just three streets that have been built without permission.
Manchester City Council says just two landlords are responsible for 14 of the illegal extensions on buy to let properties in the Levenshulme neighbourhood.
To avoid hefty fines, the landlords have nine months to knock down the extensions and return the homes to their original condition - if they do not comply, they could face fines of up to £20,000 a property and the court can order the repayment of any rents collected from tenants.
The offending properties are concentrated on Braemar Road, Brailsford Road and Moseley Road, Fallowfield.
The notices were issued following complaints from neighbours about poorly designed first floor rear extensions, says the council.
The property owners must comply with the notice or launch appeals to the Communities and Local Government secretary of state for retrospective planning permission.
Councillor Nigel Murphy, Manchester City Council's executive member for the environment, said: "It's very unusual to have so many unauthorised extensions in such a concentrated area, and this has had a real impact on the appearance of the neighbourhood.
“We encourage investment in properties which improve the look of our neighbourhoods and we ask developers and homeowners to first contact our planning department to discuss any proposed alterations. However, we will always follow this action when developers build extensions that are poorly designed and have a negative impact on a neighbourhood."
Many of the extensions have been added to houses in multiple occupation (HMO) let to students or single renters.
If the shared homes do not have HMO licences, the owners could face prosecution for illegally letting the properties.