Interior designer Caroline Riddell's London mews house is packed with ideas to copy

I'll come clean. Caroline Riddell is an old friend of mine who I have watched with admiration over the years, designing and decorating homes for clients as well as her own family. She has that knack, I've always thought, of being able to combine a great sense of style with an easy-going attitude that makes a house feel 'designed' but not at the expense of familiarity or homeliness.

When she rang to tell me she was putting her Hammersmith house on the market, frankly I thought she was mad. I loved that house. But now, sitting in her new mews house - only a few streets away from the old one - I see why she felt in need of a change.

The previous house was tall and thin and her existence with husband, two children, au pairs, office assistants, cats and dogs was altogether too vertical. It was time to refocus, to think anew about the balance of life. The proceeds of the sale enabled them to buy a derelict longhouse in Suffolk - which she is currently restoring and to where, in due course, the family intends to decamp more often - and a London property that had been the two-storey, open-plan office space of an architectural practice.

You needed vision to see the potential, but Caroline and her husband, James - who fortunately runs a successful building company, Labatt Construction - have plenty of that. First and foremost it was the location that appealed: a tiny, industrial, cobbled mews which had been the original setting for Steptoe and Son's yard in the television series, and is now a collection of offices and studios, occupied by day but deserted by night. The building the Riddells had their eye on came with planning permission for residential redevelopment on the basis that one-quarter of the property remained for 'business use'. The perfect solution. They would create office space for both their businesses in the majority section of the ground floor, with separate access to their living quarters on the first floor, on to which they would add a smaller second floor for the children.

The joy of such an arrangement - apart from the obvious one of being able to close the doors on their offices each night - was that their domestic lives would become more lateral. The winning feature of the new house for them is that, having climbed the tiny staircase bedecked in riotous Liberty wallpaper, they reach the principal room, where the ceiling opens to a raftered, double-height pitch. Here, as throughout most of the house, walls are clad in off-white tongue-and-groove boards, and parquet flooring provides a seamless transition from one room to the next. The plain backdrop allows for bursts of colour: the yellow, hand-dyed linen blinds from Susan Deliss, the sofa in rusty-orange velvet from Etro, the red-striped cushions from Colony and Farrow & Ball's deep 'Hague Blue', which has been applied to kitchen cupboards and doorways. As kitchen, dining and sitting room are all in one, this is the hub of the house - a place where furniture old and new sits happily side by side. While Caroline is an ardent antiques collector - for clients as well as for her own home - she is planning to launch her own range of furniture in the next year or so to complement the interior-design business she set up nearly a decade ago.

If the principal room maintains the proportions of the building's previous existence, dividing the rest of the space to create bedrooms and bathrooms would, you might imagine, prove more challenging. 'Did you have an architect?' I ask her. 'Oh no,' she says, 'James and I planned it out on the back of an envelope and his building team got to work.' Within the original open-plan space - formerly part of what is now the principal room - the Riddells have created a main bedroom with an en-suite bathroom and a small study from which rises a narrow staircase to the top floor. This has been cleverly extended into a substantial dormer to allow for two small children's bedrooms and a second bathroom. It's a masterclass in space management.

Nonetheless, there's a part of me that wonders if the children, now aged 20 and 18, feel a little cramped. 'Amazingly not,' says Caroline, 'they love their cosy rooms.' And, as she quite rightly points out, they have a comfortable television room on the ground floor where they can be with their friends and, in summer, the whole courtyard of the mews at their disposal. There are tables and chairs outside, tubs of geraniums and numerous potted plants awaiting the arrival of warmer weather.

In the meantime, the dogs are relaxing on the sofa, the cat is sitting on the dining room table, a Tube train rumbles by on a bridge outside and sunshine pours through the windows, flooding the house with natural light. Nothing seems out of place, yet nothing seems contrived either. I'm deeply comfortable and I'd like to stay all afternoon, but Caroline has clients to meet and a house in Suffolk that's demanding her attention - and I have a desk waiting for me.

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Caroline Riddell is a member of The List by House & Garden, our essential directory of design professionals. See the studio's profile here