In a Portuguese garden.

Making galleries is difficult: some overwhelm their contents, others are so bland that they lack architectural presence. Siza masterfully strikes a balance in this new museum for contemporary art in his home town.

Alvaro Siza de Vieira's latest building is Oporto's Museum of Contemporary Art. Built in the grounds of the Quinta de Serralves (a quinta is a country house surrounded by its grounds), the museum is completely separate from the grand pink 1930s villa which served for some years as the exhibition building. The architects were keen to connect harmoniously with the rest of the garden, and to reduce the considerable bulk of the new building, so that it relates to the three main areas of the estate: formal gardens, woods and the still farmed meadows - 'While at the same time asserting its autonomy', which Siza believes essential.

The site was chosen to spare mature trees, and the building takes its roughly north-south alignment from the paths of the old vegetable garden (which has been moved to another part of the estate). As usual with much of Siza's latest work, the exterior is modestly and finely honed, with precise white cubical forms on a stone plinth: a similar strategy, abstracted from tradition, to the one he adopted in the church at Marco de Canaves (AR August 1998, p60).

Though the site falls some 9m from north to south along the length of the building, the museum has one essential roof-line, so the building is a good deal higher at the southern end, where it bifurcates to offer a green court to the sun and the rest of the park. Hence, overall planning strategy is very clear: entrance is at the top of the site (near where people come in from the road), and gallery spaces are downhill. The 290 seat auditorium is between the museum proper and the northern edge of the estate, so that it is easily available to audiences when the galleries are closed. It is connected to the galleries underground. A graveled entrance court is formed between auditorium and entrance front of main building.

You enter on foot from the estate's northern corner (unless you are a member of staff or a special guest who can drive down to the two-storey underground car park). As is often the case with Siza, approach and entrance, though by no means obscure, are somewhat oblique. You walk down the long shady west wall from the entrance court, past the auditorium, to arrive at a porch which receives you into the corner of the main building. From here, you are drawn crabwise by a great splash of luminance to the focal space, the top-lit central atrium. Here, the essentially symmetrical nature of the main parti begins to reveal itself, but there is a certain coyness about the nature of the plan. For instance, the upper (cafeteria and general purpose) floor is reached by a symmetrical stair which starts in the middle of the south wall of the atrium and takes you by one dog-leg or the other to the gallery which surrounds the space. But the west dog-leg is masked as you come in by a wall, so your attention is focused on the left-hand stair, and on the unconnected flight which goes straight down into the library foyer. Approach to the main galleries (on the same level as the entrance) is through a large portal between the stairs.

Some of these moves recall Siza's Galician Museum of Contemporary Art (AR October 1994, p 68) where the relatively constricted and modest entrance was in a corner of the building and the plan was organized round a generous and luminous central space. In Oporto, the site was less constricting than at Santiago de Compostela, so the building was able to have a more relaxed and almost Classical layout. But it is difficult to believe that Siza is not slightly embarrassed by a formality which grows perfectly naturally out of a desire to open windows in some of the galleries to light brought in by the grass-floored southern court. Yet he is sometimes prepared to imply symmetry, even where it need not exist: for instance he signals the separate entrance to the long thin bookshop on the entrance front with a bay. On the other hand, he does not hesitate to introduce inflections from the main axis, for instance by twisting escape stairs, or by making angled hoods over gallery windows. In the larger galleries, the inverted table-top lighting device invented at Santiago de Compostela is adopted, but with refinements of section. Essentially, light is modified by striking down from a central lay-light onto a white plane which hovers below general ceiling level so luminance is reflected to the top of the walls and back to the ceiling, making for gentle indirect daylighting throughout the cool space.

Siza is not a believer in totally restrained galleries. He argues that 'Designing galleries for contemporary art requires the deliberate making of spaces which are open to different activities, especially to temporary exhibits. This does not mean ... that the autonomy of the architecture is compromised. The space itself should be open, but not neutral; a space with character, providing the possibility of balance between the things exposed and the space itself. He admits to being influenced by the Casa de Serralves, the old pink house in the middle of the estate where, in its time as a museum, exhibits interacted with the varied domestic spaces in unexpected and mutually enriching ways. Hence the great variety of spaces in his new building. There are differences in size; in height and floor level; differences between galleries in enfilade or arranged as separate volumes; differences between galleries that can have views and ones with imperforate walls. Yet, for all the variety, there are no clumsy moments (even if circulation is a bit tortuous at times). There are only about a dozen galleries in the building (depending on how you count), but they offer curator and visitor a great wealth of opportunities for interpretation - and inspiration.

Architect: Alvaro Siza Arquitecto, Oporto

Design team: Clemente Meneres Semide (principal architect), Tiago Faria, Christian Gaenshirt, Sofia Thenaisie Coelho, Edison Okumura, Abilio Mourao, Avelino Silva, Joao Sabugueiro, Cristina Ferreirinha, Taichi Tomuro, Daniela Antonucci, Francesca Montalto, Francisco Reina Guedaes de Carvalho

Structural engineer: GOP-Joao Maria Sobreira

Mechanical engineer: GET-Alfredo Costa Pereira

Landscape architect: Joao Gomes da Silva

Photographs: Duccio Malagamba & Christian Richters