Work will start on the £35m Shoreditch job in November at 18-30 Leonard Street.
The site was acquired from Derwent London and AHMM Architects has drawn-up plans for a mix of 47 one, two and three bedroom apartments and 19,000 sq ft of commercial space.
London Square has also purchased a site on Upper Richmond Road from Briargrove Limited.
The developer will be creating a £40m scheme totalling 68 homes with commercial space on the ground floor.
Brian Betsy, Land Director of London Square, said: “Both Leonard Street and Upper Richmond Road are prime sites in strategic locations that fit the London Square profile – highly coveted London areas where supply is continuing to outstrip demand.
“We are looking to buy more key sites in locations like these over the next few months.”
The deals mean London Square has six prime sites on the go.
The firm was set-up last summer by former Barratt London regional chairman Adam Lawrence.
Lawrence said: “These sites are what London Square is all about – creating excellent schemes in sought-after areas where people want to live and creating individual homes designed for real lives.
“We now have six prime sites in total, including Wimbledon Village and Fulham, plus other significant opportunities in the pipeline.
“We have delivered these in 12 months since launching London Square, demonstrating our agility, sound financial backing and a first-class management team in place.”
The company’s plan is to reach a target of building over 600 new homes annually within the next five years.