125 Park Road was completed in June 1970 for the Mercury Housing Society. The Mercury Housing Society developed from a co-ownership scheme involving Sir Terry Farrell where the 40 co-owners who lived in the building also collectively developed it. The 10-storey block for the Mercury Housing Society provided 18 two-bedroom flats, 18 one-bedroom flats, 4 ‘penthouse’ one-bedroom flats, a caretakers flat and basement laundry.

The plan form developed out of basic ideas on flexibility. It consisted of a central core with multi-access points to a surrounding ‘living’ zone. This zone, free of structural walls and with continuous perimeter glazing, heating and electrical sockets, allowed maximum flexibility of sub-division. External cladding reflected this need for flexibility, with a regular grid capable of accepting a wide variation of internal layout, whilst maintaining good views out of the building.

The block was designed without balconies, but mid-way through the design process the Farrell/Grimshaw practice persuaded the Housing Corporation on the cost-effective impact of adding an extra two feet of space around the whole building, making all the flats more generously sized. Corrugated anodised aluminium cladding was also selected, to allow for flexibility in curved design, whilst being lightweight, aesthetically acceptable and economically viable. One of the main design exercises was to get as many living rooms as possible to have a good view of the park. Rather than compromise with what the architects termed ‘oblique glimpses’ it was decided that three of the four flats on each floor should have a straight-on view of the park; the fourth, whilst having good orientation to the sun, would not overlook the park at all and this would be reflected in the rent. The result was a simple plan form in which the block was flanked by the 2 two-bedroom flats each running from front to back of the building, with a large living room overlooking the park and the sun to the rear. A single bedroom flat occurred between these two flanking flats, one overlooking the park and one overlooking the Marylebone Goods Yard.