The former day centre in Harlington Road West, Feltham, was home until recently to Age UK, and Hounslow Council had agreed only last November to sell the building after the charity moved out.
But council leaders now want to keep the property so it can replace the existing register office in the grounds of the Civic Centre, in Lampton Road, Hounslow.
The entire Civic Centre site is due to be sold for housing as the council moves to a smaller HQ closer to Hounslow town centre.
Subject to approval at a cabinet meeting on Wednesday (May 20), the council will borrow £520,000 to refurbish Feltham Lodge so it is ready to record births, deaths and marriages from July 2016.
A report by council leader Steve Curran states the service generates a "substantial income" and moving it to the west of the borough is suitable as 84% of customers currently come from the Hounslow, Feltham or Bedfont area.
Bedfont Library and Chiswick Town Hall were both considered as alternatives, according to his report, but were ruled out as they are both currently used and the latter is already licensed to host marriages.
The council wants building on the Civic Centre site to begin next summer so it qualifies for up to £15m of grants which it says would help meet its target of making 40% of the new homes there affordable.
The council's new offices are set to open in 2018, with a car park in Bath Road, just off the High Street, the preferred location.