The best new building in London, according to the Royal establishment that bestows such honours, is not one of the City’s jauntily sculpted skyscrapers, nor a glitzy cultural beacon, nor even a flagship development of desirable homes. It’s an old shoe polish factory, converted into an unassuming charity office building, tucked away on a side-street in Vauxhall.

The Foundry, designed by 36-year-old Lynton Pepper of small practiceArchitecture 00, punches well above its weight in both architectural ingenuity and the social impact that it will bring, and was a surprise winner of Riba’s London building of the year award.

Developed by the Ethical Property Company, with Barrow Cadbury, Lankelly Chase, Trust for London and Joseph Rowntree Charitable Trust, it provides a new home for 25 different social justice and human rights organisations whose work tackles everything from renters’ rights to solar-energy aid and the rehabilitation of addicted prisoners. Now brought together under one roof, in a loose-fit building specifically designed for discussion, collaboration and exchange of ideas, their work, says the company’s director Susan Ralphs, can be much more than the sum of their parts. And it doesn’t do them any harm to be working in slick new premises that exude a level of gravitas and professionalism greater than the £20 per sq ft rent would suggest.

“After just a few months of being here, some of our tenants have already said the building gives an extra weight and confidence to their organisations,” says Ralphs. For just £5.2m, an extraordinarily tight budget for a project of this size, the architects have crafted a place that speaks more of angel-funded start-up incubator than shoestring non-profit hub. “It says something about them to visitors and potential funders,” she adds. “It shows they’re serious.”