A playdate with Rembrandt at the revamped Dulwich Picture Gallery

The design of the building, by the brilliant, innovative and somewhat crabby Sir John Soane, includes a mausoleum for the Swiss and French art dealers who founded the collection; its shallow dome was an inspiration for those of the now battered K2 and K6 red phone boxes that were everyday icons on British high streets. The gallery building is a work of seriousness and an object of reverence. In places, with its largely windowless brown brick walls and austere rebuilding after wartime bomb damage, it is a little drab – a manilla envelope, if a distinguished one, for its gorgeous contents.

The challenge of the new ArtPlay Pavilion, the centrepiece of a £5m makeover of the museum’s grounds, was to create a place in these sober surroundings where under-eights (and sometimes older children) can, through imaginative play, take maximum delight in visual art. Here, child’s play meets high architecture, gaudy meets gravity.

How to make an august institution available to young minds, without talking down? In this task, a team led by the architects Carmody Groarke, known for a subtly dramatic museum of boats in Windermere and the temporary wrap they put around Charles Rennie Mackintosh’s Hill House in Helensburgh, near Glasgow, have triumphantly succeeded.

The pavilion stands in the small park that is part of the gallery’s original concept, open to the public whether or not they’ve come to see art – a nice enough spot but formerly a bit flat and unimaginative. Now, with the landscape architect Kim Wilkie, almost half a hectare (about an acre) of previously fenced-off ground has been made into the species-rich Lovington Sculpture Meadow, which includes a curving, plait-shaped earthwork inspired by Rembrandt’s Girl at a Window – one of the gallery’s star exhibits.

Along with a new revolving programme of modern artworks, 130 trees have been planted and the crowns of existing ones raised to allow views across the grounds. The scene is richer, more animated and more unified than it was before.

The pavilion stands at a pivotal point between the meadow and the museum. It is a geometric nugget; four-square and single-storey, with a big circular window in each wall. Thin-edged metal canopies project from each side, to provide shelter from sun and rain. Its double symmetry is like that of a Palladian villa set in a landscape, or indeed the nearby mausoleum, but it is made more dynamic and less formal by being set at 45 degrees to Soane’s main building. You come across it obliquely, like a bandstand or a kiosk in a park, say the architects.

It is a thing in itself, detached and proud, but also a device for creating relationships in the surroundings. Its siting and skewed angle mean it looks towards, and can be seen from, the main entrance and other key locations. It forms a composition with the nearby keeper’s cottage, a modest building of about the same date as the gallery, now extended, refurbished and embellished with yellow awnings to make a canteen for school parties. One of those Soane-inspired phone boxes, relocated, joins the conversation.

The new work is respectful towards the venerable gallery building itself; complementary but distinctive. It is timber-framed and timber-clad with vertical, rough-surfaced, painted Douglas fir, which the architects say echoes both the brick texture of Soane’s walls and the way they are “made up of small pieces”. The boards are arranged in five horizontal bands, each projecting slightly beyond the one below, to make an inverted ziggurat. Carmody Groarke play with subtle changes of depth, with incisions in material that catch sharp shadows, which is a version of what Soane did with brick and stone.

Then there are the things the Soane gallery mostly lacks – windows – joyously realised in a shape equally fascinating to classical theorists of geometry, as in Leonardo da Vinci’s drawing of a man inscribed in a circle, and small children. From inside the pavilion, they create a sense of looking out on the world from a sheltered space. They bring a more-than-adult scale while also, by coming close to the ground at their lowest point, connecting with that of infants.

A diagonal roof structure of crisscrossing timber beams completes the feeling of benign enclosure, with a glazed opening in its centre that allows more light to descend. An ample room is made, cross-shaped in plan, within which the HoLD Art Collective have invented an eventful array of installations inspired by the works in the gallery – a miniature Venetian bridge from a Canaletto, for example, or clouds from a Poussin – and elements designed to engage all the senses, such as lavender-filled cushions and a panel that vibrates like a waterfall. HoLD’s style is less restrained than Carmody Groarke’s, but the two go well enough together.

Meanwhile, the project’s architects and engineers, and the builders 8build, have quietly but assiduously set about doing the right thing by sustainability and accessibility: a heat pump and solar panels power the gallery campus, there is step-free access throughout, and the pavilion structure is designed so that it can be made of the smaller sizes of timber provided by British trees, avoiding the energy-consuming need to transport the material over long distances.

This is grown-up architecture that works with such things as shadow and reflection, scale, and the real and apparent weight of materials; the ways they are put together, the sympathies that can be found between timber, masonry and metal. It pitches itself astutely between monument and garden shed.

At the same time it finds the sense of play underlying everything on the site, Soane and Rembrandt included, given art and architecture are adult versions of children’s games. And, by gently lightening up the old building’s severity, it takes a bit of the dull out of Dulwich.

Photographs by Luca Piffaretti