The fire wrecked the two homes in Plex Road, Hadnall, after crews were called out just before midnight on Friday into Saturday morning.
A Shropshire Fire and Rescue spokesman urged any witnesses to contact the police.
He said: "Crews were called to two properties under renovation in Plex Road, Hadnall, near Shrewsbury. Both severely damaged by fire. Fire Investigation called out and confirmed deliberate ignition. If you have any information contact West Mercia Police on 101 or Crimestoppers UK on 0800555111."
A West Mercia Police spokeswoman said: "Around 12.30am on Saturday, October 1, we received a report of a fire at a semi-detached property on Plex Road, Hadnall. We attended with Shropshire Fire and Rescue.
"No injuries were reported but significant damage was caused to the property.
"It is believed the fire was started deliberately and enquires are ongoing."
Pictures showed considerable damage to the roofs and broken windows with a fire crew standing guard.
It is understood that both properties were empty at the time.
One of the two semis is being let by property agents Monks with a rental value of £1,150 per calendar month. It is described as a "newly renovated three bedroom semi-detached house situated in this idyllic rural location on the Sansaw Estate."
It had a newly installed kitchen and bathroom. The properties sit off a single-track rural road and are some distance from the nearest buildings.
Shropshire Fire & Rescue Service sent two fire engines from Shrewsbury. They used one hose reel jet, one aerial ladder platform and small gear to deal with the incident. They confirmed that there were "no casualties at this incident".
The incident stop message was received by fire control at 2.04am on Saturday.
A replacement fire crew was on the scene on Saturday as investigations continued.