Properties like 900-year-old Tretower Court and Castle, near Crickhowell, as well as private homes dating to the medieval period are to be investigated.
The oldest domestic house identified so far is Hafod-y-Garreg in the Wye Valley which dates to 1402.
The study will take timber samples to pinpoint buildings' ages using tree ring dating or dendrochronology.
The Royal Commission on the Ancient and Historical Monuments of Wales (RCAHMW) is working on the project looking at properties in the old county of Breconshire in partnership with county history society, the Brecknock Society, which is particularly interested in properties in the Usk and Wye valleys.
It is due to get under way in the New Year with the findings published some time later.
Other properties being tested are The Three Tuns pub in Hay-on-Wye which is a 16th Century timber frame building.
The surviving wooden gates at 12th Century Hay Castle are also to be tested.
It was built by the powerful Norman Lord William de Braose and was ransacked the following century before being rebuilt by Henry III.
Tree-ring dating has already shown that Castle House, the adjoining Jacobean mansion, which was severely damaged by fire in 1939 and 1977, was built earlier than 1660.
RCAHMW architectural investigator Richard Suggett said the study would help people to understand the chronology and development of buildings in the area.
A study to prove the age of the oldest buildings in parts of Powys is being launched by experts.