Another property being built by Bovis Homes as part of the redevelopment of the former St Margaret's Hospital at Great Barr was also set ablaze.
Firefighters from West Bromwich station were called out to the scene.
West Midlands Fire Service said: "There were two houses on fire virtually opposite each other.
"The showhome was only about two weeks from being completed and the hall and staircase were gutted.
"The other house was about half-built."
"There is security on the site and it appears an offender, or offenders, broke in to start the fires."
Police were investigating today.
Written by Evening Mail; Birmingham
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