Middlesex's Pro40 play-off match against Northamptonshire on Sunday, September 23 will be staged at Southgate.
It had been reported that the game might be held at Lord's as opposed to Southgate, an outground with a limited capacity.
But an MCC spokesperson told Cricinfo that Lord's was unavailable. "Our winter works programme begins the day after the Cockspur Cup final, and this was already delayed by a day because of the floodlit fixture [on September 10] being switched to Lord's from Southgate.
"We play more cricket at Lord's than probably any other first-class ground in England and so Mick Hunt has to get on with the works the minute the season finishes. The square was top dressed last Wednesday."
The play-off starts at 12.45pm and the gates will open an hour and a half earlier.