There is real, good new architecture to be found in this river side of Somerset House, opening onto the Embankment through Chambers' original arched watergate. The architecture here is by Peter Jenkins of Inskip and Jenkins, and it is an object lesson in how to adapt and reveal a historic building in a wholly contemporary way. He has understood the spirit of the interior of Chambers' Embankment building. It was originally a bargehouse, open to the river until the Victorians narrowed and canalised the Thames by throwing the Embankment - a giant conduit for sewers and underground railway lines with a wide road on top - across the front of it. There is no getting rid of the Embankment, but Jenkins has done much to recreate the feel of the river entrance.

Now you walk straight in over a semi-transparent bridge of steel mesh and glass. Beneath your feet you see the pebbled foreshore of the Thames at its original level. Down there - at present uncomfortably marooned in what feels like a giant fish tank - is a ceremonial Navy commissioners' barge of the period, for this was the naval part of the building. The barge display doesn't work and needs to be re-thought. But up above, things get better and better. Once through the watergate, you are in a large new foyer acting as an interpretation centre for the whole building, with some compelling computer graphics explaining its evolving history. Straight ahead is a cafe and the route up through to Chambers' stately, austere courtyard - for years a Tarmac car park, now a granite-paved public square doubling as an outdoor performance venue. To your right is the entrance to the Gilbert Collection, to your left its shop.

In the original long, vaulted, double-height bargehouse that now houses much of the Gilbert Collection, Jenkins had to cram a lot in. He does this by running a timber-clad mezzanine through the space, keeping it clear of the walls by means of glass slots, and leaving generous full-height spaces at the ends. The mezzanine has a curving dark timber underside recalling the boats that once floated in here. All the details - stairs, handrails, display cabinets, lighting - are entirely modern, generally in glass and stainless steel, but also robust and confident. The sequence of glass-cube display cabinets is particularly successful, as is the Russian Orthodox section, placed in what was once a little bargekeeper's house, which is as perfectly proportioned as a church sanctuary. Other rooms, set back and up from the main run, are virtually pitch-black, the treasures seemingly floating in space as they are caught by the ultra-precise lighting. It takes a rare understanding to interpret and adapt a historic building in this way.

Jenkins' Gilbert Collection in Chambers' vaults

Above the Embankment building another set of architects - Sir Jeremy Dixon and Edward Jones, fresh from their triumph at the National Portrait Gallery - have restored the old promenade terrace, linked it via a simple glazed ramp to Waterloo Bridge, and added a demountable summer cafe. They have also designed the system of fountains in the great courtyard, which will rise (from July) directly from the paving. This is design so low-key as to pass virtually unnoticed, as is the work by conservation specialists Donald Insall and Associates, who have restored the key naval rooms and staircases in the south wing of the complex. In contrast, Jenkins' work stands out.