21 Upper Grosvenor Street was constructed in 1732. The mews was constructed at the later date in 1869 at the expense of the original large garden. The building consisted of five storeys above a basement with a two storey canted bay extension and a ground floor passage at one side of the garden connecting the house with the mews building. The building is Edwardian in character and is now three storey above ground with a mansard attic clad in Portland Stone ashlar. The ground level is rusticated above a plinth with a doorway in Doric classical columns with dosserets.

The scheme includes part excavation of the basement 1 level to form a swimming pool, alterations to the rear elevation with the removal of the fire escape stairs, and alterations to the bay window, creating access onto the rear roof at the ground floor and use of the terrace. At the third floor use of the second floor bay roof creates an additional terrace infilling the light wells at the lower ground floor level. The proposal reconfigures the basement 1 level with the pool, gym, sauna/fitness zones, cinema room, utility area, and additional guest bed spaces with storage in the pavement vaults. On the ground floor the entrance hall leads to a study with adjoining formal dining room, morning room, family room and kitchen area  and a library. On the first level additional reception areas 1 and 2 lead to the large roof terrace. On the third  level the master bedroom suite includes dressing rooms, a grand bathroom and access to a private terrace over the bay window. At levels 3 and 4 there are a further 4 bedrooms with ensuite facilities and a leisure room. 

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