Question: When was the old Scottish building first built that is roughly 28 metres in diameter ?

text: List of oldest buildings in Scotland
Main list -- Iron Age

This article lists the oldest extant freestanding buildings in Scotland. In order to qualify for the list a structure must:
Building Location Council area First built Use Notes
Dun Ringill Strathaird , Skye Highland 1st millennium BC Semi-broch Known to have been occupied in the medieval period by Clan Mackinnon the ground floor living space is only 47 m2
Old Scatness Sumburgh Shetland 400-200 BC Broch and wheelhouse Like Jarlshof the site was occupied by Iron Age peoples , Picts , and Vikings
Dun Troddan Glenelg Highland 4th century BC to 1st century AD Broch One of the best-preserved brochs , the dry-stone walls surviving to over 7 m in height in places
Dun Telve Glenelg Highland 4th century BC to 1st century AD Broch The best preserved mainland broch , with walls surviving to 10 m , located only 500 metres from Dun Troddan
Jarlshof Sumburgh Shetland 200 BC Various A complex of preserved wheelhouses , amongst the remains of a variety of much older and more recent buildings
Yarrows broch Wick Highland 200 BC Broch Situated on the edge of a loch , underwater stonework may indicate a docking area
Dun Vulan Bornais Na h-Eileanan Siar 150 BC Broch Originally 10 metres ( 33 ft ) in height , now reduced to walls of 1.52 metres ( 5.0 ft ) . A Pictish house was later built within the walls
Dun Ardtreck Minginish , Skye Highland 115 BC Semi-broch Initial occupation appears to have been brief and to have ended in violence and destruction
Broch of Mousa Mousa Shetland 100 BC Broch Amongst the best-preserved prehistoric buildings in Europe
Ness of Burgi fort Sumburgh Shetland 100 BC Blockhouse fort Surviving to only 1.5 m in height , the blockhouse was once 22m long , but has suffered from cliff erosion
Midhowe Broch Rousay Orkney 1st century BC or older Settlement Lies opposite the Broch of Gurness overlooking Eynhallow Sound
Broch of Burrian North Ronaldsay Orkney 1st century BC Broch May have had three distinct phases of occupation
Dun Beag Bracadale , Skye Highland Late 1st millennium BC Broch Visited by Boswell and Johnson in the 18th century
Mine Howe Tankerness Orkney 100BC - 110AD Souterrain Once thought to be a broch , it was used for metal working and may have had a religious purpose
Broch of Gurness Evie Orkney 60 AD or older Settlement Oldest confirmed dates are Roman pottery from 60 AD and radiocarbon dates for the late first century
Dun Mor Vaul Tiree Argyll and Bute 60 AD Dun The original structure dates to c. 445 BC . A fragment of a Roman glass bowl made in the Rhineland between AD 160 and 250 was discovered in the interior
Broch of West Burrafirth East of Sandness Shetland 100 AD or older Broch The stone stands eight or nine courses high for much of the circumference
Dun Fiadhairt Duirinish , Skye Highland 200 AD or older Broch A clay model of a bale of wool , believed to be Roman , may reflect trade between the two cultures although a Roman fleet is known to have circumnavigated Scotland in the 1st century AD
Edin 's Hall Broch Abbey St Bathans Scottish Borders 100-140AD Broch Broch believed to have been constructed between the two Roman occupations of southern Scotland , on the site of a much earlier fort
Broch of Borwick Yesnaby Orkney 200 AD ? Broch On a cliff promontory , the seaward wall is badly eroded