Opening next door to Patty & Bun's own site on James Street, the 50-cover Sidechick restaurant will be split over two floors, with dining rooms on both levels and an open kitchen on the ground floor.
An outdoor terrace area will seat an additional 20 covers.
Property agency Shelley Sandzer secured the site on behalf of the brand.
Sidechick launched last year as a delivery-only brand, and initially operated out of Patty & Bun's central kitchen in east London.
The restaurant menu will be 'small and simple', focusing on quality, seasonality and flavour.
Corn-fed roast chicken will be central to the offering, available alongside a range of seasonal vegetables, salads and flatbreads.
“This site is absolutely perfect for Sidechick’s first foray into bricks and mortar dining, next-door to sister restaurant Patty & Bun Marylebone, also a debut location, having opened back in 2012," says Sidechick and Patty & Bun founder Joe Grossman.
"The overwhelmingly positive response to our delivery-only service during the spring-summer 2020 lockdown encouraged us to source a permanent location for the concept, which Shelley Sandzer have helped us secure.
"Hospitality faces an uncertain future, so we are doing all we can to create jobs and bring diners new concepts to look forward to in the coming weeks, months, and years.”