Townsend transfused: an extension to one of London's best loved smaller museums respects the architecture of the original, and makes sense of circulation and connection with surrounding gardens.

The Horniman Free Museum in Forest Hill in the southern reaches of greater London is, like the Soane Museum, one of London's very great pleasures. Set into the steep slope of the old London Road, the building was designed by the Arts and Crafts architect, Charles Harrison Townsend, at the turn of the nineteenth century to house the collections of tea merchant and traveller, Frederick John Horniman. As were those of other merchant adventurers, past and present, the collections are eccentric and intensely personal, comprising anthropological curios as well as exotic zoological specimens. The collection has been ordered in modern times, but even so you can glean a sense of the collector's marvellous promiscuity as you move down the length of its two halls, from African masks to a stuffed walrus.

Townsend had come to Horniman's attention with his design of the Bishopsgate Institute and Whitechapel Art Gallery and his building, with a 1911 annexe known as the Emslie Horniman hall, contributes to the delight. For his new client he contrived a castle-like structure of Doulton stone and brick standing back from and elevated above the London Road. A curved gable indicates the barrel-vaulting of the two galleries within, on plan set like railway carriages one behind the other. A massive stone clock tower' with rounded corners and pinnacles stands sentinel over the arched entrance. Across the face of the building, a mosaic by Anning Bell presents a vision of Humanity in the House of Circumstance, a moment that prefigures the 'recreation, instruction and enjoyment' that Horniman wished to visit on people coming to see his museum. Increasing the pleasure of the place are 16 acres of well-tended gardens spreading across the hill behind and containing a fine example of a Victorian conservatory, now restored.

Overtime, the museum developed in an ad hoc way, acquiring unsatisfactory extensions -- workshops, an education block and west hall -- all of which created access problems and compromised the integrity of the original buildings. Further along the London Road, next to the gardens' entrance, is a curious hairy relic of the 1970s. Designed by Archetype for CUE (Centre for Understanding the Environment), it was always intended to be temporary.

Allies & Morrison's scheme, funded by Heritage/Lottery money, cleared away the various accretions and tidied up the old museum buildings. On an L-shaped site, their new building stretches back from the London Road to the Victorian conservatory on the north and is bounded on the south by the Emslie Horniman hall and, on the east, by Townsend's halls. The most visible part of the scheme is a long barrel-vaulted structure facing onto the road and gardens.

As usual with this practice, architectural clarity belies the fact that the building does a number of complex things simultaneously. In the first place it increases the museum's space by about 50 per cent, providing new galleries -- for an impressive exhibition (elegantly designed by Ralph Appelbaum Associates) of musical instruments, large touring shows, and less fragile objects that can be handled -- as well as an education centre, a shop, and cafe. Linked to the historic core, it simplifies circulation and provides access for disabled visitors, previously inhibited by Townsend's precipitous steps. It also connects the museum directly to the gardens for the first time, thereby realizing Horniman's original intention. As a visitor, you now experience the gardens from inside, drawn as they are like a mantle around the buildings and visible through Allies & Morrison's transparent walls.

If the historic core imposed its own imperatives, so did the site which slopes up from east to west and south to north. Slotted in between Emslie Horniman and CUE, the south face of the new building completes an entertaining series of four, each one slightly stepped back from its neighbour as the London Road rises. Employing the same Doulton stone, the architects have referred to the roof forms of Townsend's two buildings by compressing them into one. (Externally, the solid triangle within the curve is visually a trifle awkward, and works better when seen from inside; for instance in the cafe, where light streams through glass panels set into the curved interstices over a vaulted ceiling.) An upper glass bay (a Voysey echo) diffuses light into the long music gallery beneath.

Allies & Morrison's new entrance is on the west and approached from the gardens. After turning into the main garden avenue from the London Road, you see once again the buildings laid in receding layers. Red brick on the extension's west face, under the silver vault of the roof, picks up on that used by Townsend for the north and west elevations of the north hall, and the new brickwork is subdivided into panels to reflect the masonry piers of the old structure.

A stone path from the main garden avenue delivers you to the main upper floor of the museum (level 2) and into the cool, white interior of the building. To the left is the vaulted cafe, glazed on three sides; to the right, a shop and classrooms.

But the fulcrum of the whole scheme is a double-height void sunk into the ground alongside, at the level of Townsend's south hall. A great deal is going on here. On level two, you are connected, through a glass wall on the north, to the extravagant vision of the conservatory floating at the end of a terrace, and, through a new door, to Townsend's north hail and south gallery balcony. At the lower level are the new galleries and entrance to the south hall. The internal courtyard itself will be another exhibition space.

Floors of oak and stone, white walls cut by slots to the exterior and washed by light, large areas of glazing and thoughtful detailing together compose a vocabulary that is familiar from past work by this practice. The architects have made much of the level changes, cutting slots to give unexpected views of neighbouring roofs or sky, and (as in the case of the internal courtyard) compressing volume to make its expansion all the more surprising. Undoubtedly, this well considered and thoroughly agreeable building, which quietly respects Townsend and the site without self-indulgent dramatics, will endure and, if the CUE building is ever demolished, will be expanded.

(1.) The clocks were for the benefit of the poor who could not afford timepieces.