The National Portrait Gallery is to install full disabled access through its main entrance and studio and exhibition spaces.
The location of the site, tucked behind the National Gallery, means it has a number of level changes and angles, making access a challenge for people with limited mobility.
The installations will come as part of a £36 million project to reorganise the set-up in what is considered the most ambitious undertaken by the gallery.
It has also won a £9 million heritage lottery grant to help pay for a significant expansion of its exhibition space, by repurposing areas largely used as offices for more than half a century.
The gallery already expects a further £7 million in its coffers and is due to appoint an architect this autumn.
Around 2 million people visit the gallery each year and this is expected to rise as it expands by 20% and improves access.
Director Nicholas Cullinan told the Guardian: “This major grant enables the biggest transformation the gallery has ever undertaken. We are going to make ourselves an essential place for everyone to feel part of the culture they have been born into, chosen, or are seeking to understand, to become a truly national gallery for all.”