Stratton & Reekie's six staff, including co-founder Amanda Reekie, who will become a senior consultant at ING, will move over in April.
The terms of the deal were not disclosed, but ING founder and MD Leanne Tritton will remain the agency's sole owner.
ING currently has 30 staff and is based in Shoreditch. Key architecture clients include Adjaye Associates and AL_A, while the property division of the business works with developers HB Reavis and Native Land, among others.
Tritton said: "This new arrangement means both firms can offer their clients an enhanced range of services, as well as the chance to work with one of the most expert teams to be found anywhere in the built environment and property sector."