Arguably the two most important aspects of any museum are the quality of its collection and the calibre of its staff. Wolverhampton Art Gallery is fortunate enough to be well served in both areas.

Its excellent array of 18th-century art, inventively hung in the Georgian Gallery, is one of the best examples of fine art interpretation I have seen. The layered information is accessible while the five innovatively designed interactives mean that the hang caters for families and connoisseurs.

And the gallery has recently acquired 376 sq metres of new space thanks to a £6.7m extension. The museum opened in 1884 and the adjacent school of art was incorporated into the museum to mark its centenary. It is the space in between these two buildings that has now been filled.

Instead of an undignified area known as the 'back yard' there is now an arrestingly modern structure plus a neat courtyard linking the museum to the Makers Dozen artists' studios.

The view from Wulfruna Street is an impressive one: the angular mix of terracotta, glass and steel manages to both differ from, and harmonise with, the attractive red brick of the old arts school. The designer of the new building, Bristol-based Niall Phillips Architects, specialises in producing solutions for historically sensitive areas.

The extension increases the provision for education, adds function and meeting rooms and, most importantly, provides additional gallery space. This will house not only touring exhibitions but also the gallery's outstanding collection of pop art - a legacy of the farsightedness of its first curator, David Rogers. This, plus the unique Northern Ireland Collection, means that the gallery's holdings, while eclectic, are coherent and have strength in depth.

This promises to continue: the Northern Ireland Collection, begun in the 1980s, has recently been supplemented to include powerful works such as Rita Duffy's Veil (2002), formed out of six cell doors from an abandoned women's prison in Armagh. Both it and other works on a similar theme are on display in the old part of the building until the beginning of next year.

On the ground floor of the new extension Pin Up: Pop Art and Popular Culture is on until August. The museum's excellent collection, enhanced by loans, looks striking in the icy blue, music-filled gallery. London design consultancy Airside was commissioned to provide the seating and arrange the computer interactives. The latter builds on an oral history project conducted in 2005 in collaboration with the University of Wolverhampton.

This gives the exhibition a local flavour and relevance. My visit to the gallery coincided with a visit by pupils of Wolverhampton Grammar School who seemed to thoroughly enjoy the art and the interactives, including the chance to try on 1960s-style clothes.

Pin Up is the first of four themed hangs to be organised
over the next two years. These aim to give a more nuanced account of pop art instead of the standard 'whole story' approach. The awkward shape of the space must have been a challenge.

The triangular gallery, as the name suggests, is just that. This is partly a result of the restricted plot and partly because, rather than an unremarkable extension, the museum opted instead for a 'landmark' building. This clearly took precedence over curatorial considerations.

Spectacular though it is, the angular space is far from ideal from a curator's point of view. It will surely suit some exhibitions more than others: Korean artist Choi Jeong Hwa's site-specific pop-inspired show, which opens this month, is likely to look fantastic, not least when the lighting underneath the atrium's suspended walkway is illuminated, bathing the main foyer in changing colours. But the staff will need to think hard how to make best use of the space in the future.

One thing that can obviously be improved is signage. The sort of pared-down minimalism suitable for an architectural journal seldom meets the needs of a museum visitor struggling to navigate a building.

The linkage between the old and new parts of the museum also needs attention - the dark wooden doors on the ground floor form a barrier, confusing visitors as to where they should and should not go.
Entrances are clearly problematic.

In places the doors appear too heavy for their hinges, with signs asking visitors to 'open doors gently'. There is a more serious predicament at the new entrance. At night this is closed off by a large, folding metal screen.

Unfortunately, faulty construction has prevented this from opening fully, leaving just a narrow, unappealing gap to slip through. It is strangely reminiscent of Rita Duffy's Veil and is not the bold welcome envisaged by the museum. Even when this is rectified there will still be a problem. The large window wall of the new facade has had to be screened off to allow more room to hang the pop art collection.

This is a far cry from the airy, open structure implied by the architectural model on display in the museum. It threatens to turn the extension into an underused back entrance rather than its much vaunted '21st-century face'.

Another criticism is the flight of steps leading to the concealed doorway. This was a missed opportunity to provide a fully accessible entrance - meaning less able visitors must continue to use the side door off St Peter's Gardens.

The overriding objective for the new facade is to connect the museum with the institution situated directly across the road. The triangular gallery literally points to the main building of the University of Wolverhampton.

The gallery will need to convert these students into repeat visitors given that its target number for 2007-08 is 190,000 - an increase of some 32 per cent on 2004. It deserves to achieve or better this goal.

Corinne Miller, the head of arts and museums at Wolver-hampton, has said that the gallery sustains the original vision of the founding fathers, who believed that art can change peoples' lives. Given the quality of its staff and collections, this seems justified. But it's a shame that this transformation can only take place from Monday to Saturday, as the whole place is shut on Sundays.

Stuart Burch is a lecturer in heritage studies at Nottingham Trent University

Project data

Cost: £6.7m

Main funders: Heritage Lottery Fund £4.95m, Advantage West Midlands, Wolverhampton City Council, Capital Challenge, Single Regeneration Budget, European Regional Development Fund, Esmée Fairbarn Foundation, Regional Arts Lottery Programme

Architect: Niall Phillips Architects

Main contractor: William Sapcote & Sons

Identity, interiors and interactives: Mae Architects, Airside