The £11m deal will see the firm convert the office block into residential apartments just 200 metres from Sevenoaks station.
Construction work will start this summer on the scheme with a development value of £40m.
Andrew Telfer, divisional chief executive at Willmott Dixon’s development division Regen, said: “This is another example of the versatility and scope of our development capability.
“Tubs Hill House very much reflects Prime Place’s ethos to provide high quality homes in popular locations with good transport links, and we are looking forward to giving a new lease of life to this building once the various permissions have been achieved.
“It is also a further statement of Willmott Dixon’s development ambitions to deliver homes for both sale and private rent at scale through our Prime Place and be:here brands.
“This already includes developing London’s first purpose-built community of private renters as part of our 1,176 home Aberfeldy Village in Tower Hamlets, plus the forthcoming Brentford FC masterplan that combines homes for sale and private rent. ”