The architect's reworking of the Building Centre Trust's headquarters kicked off just before Christmas.
The new space for the charity will contain seminar rooms for 220 people, conference facilities, a new product information centre, new trade exhibition stands, cloakrooms, toilets and office space.
This work comprises the second phase of a drive to overhaul one of the design and construction industry's main centres in the capital.
According to the architect, before work began on the refurbishment - which included an earlier phase involving the creation of the New London Architecture gallery - the building was 'tired'.
Work on the new facilities is due to be completed by the end of April.