Thermae Bath Spa

Bath's hot springs have been used since Roman times: from AD 43 the Romans developed Aquae Sulis as a place of rest and relaxation, and in AD 70 they built a reservoir around the springs, then a series of baths and a temple to the goddess Sulis Minerva. In 1590, the charter by which Elizabeth I made Bath a city laid down that the thermal waters were to stay open to the public in perpetuity.

The Hot Bath closed in 1976 and the Beau Street baths in 1978; from the 1970s until 2006 the spring water ran away into the Avon. In the 1980s, five bids by commercial consortia to reopen the baths foundered on the cost of restoration.

In 1995 the new Bath and North East Somerset Council, owner of the spa buildings and guardian of the thermal water, was encouraged to seek National Lottery funding; it applied in 1996, and on 13 November 1997 the Millennium Commission awarded the Bath Spa Project £7.78 million. The brief called for a new building of its time, to celebrate the third millennium. More than 140 practices put forward ideas; Nicholas Grimshaw's was chosen for its innovative use of materials. Acknowledging that it did not specialise in restoring old buildings, the practice brought in Donald Insall Associates for that part of the scheme.

The 1920s Beau Street baths were demolished in summer 1998, which allowed archaeological excavation on the site of the future building. The opening was delayed, largely by the complexity of the project and the difficulty of working with Grade I listed buildings on a very restricted site; the archaeology took a long time, and the scheme went through planning three times, once to add space for the facilities, so the building ended up a fifth larger than first designed. The spa opened to the public in August 2006.

The New Royal Bath stands among five existing buildings under strict protection, the Cross Bath and the Hetling Building among them. It is a free-standing glazed volume that follows the line of Bath Street and matches its older neighbours in height and material. At its heart, a cube of Bath stone echoes John Wood the Younger's Hot Bath. The idea was to keep the solid mass of the new building apart from the old ones while linking them all in a very transparent way: a translucent envelope wraps the cube, and glass walls and footbridges join the buildings.

The cube rests on four pillars, which made room beneath it for a large indoor pool, the Minerva Bath, whose free-form outline contradicts the geometry of the cube. The glass envelope lightens this pool despite the mass of stone looming above, and the open volume helps the air circulate. Circles and curves play against the cube's straight lines: where the tops of the pillars meet its base, in the portholes cut into the cube, and in the rooftop pool, whose circles form the whirlpools. The Rotunda, a detached stair tower clad in Bath stone, is another circle; it houses the services and the water circulation.

The building is extensively glazed, and frameless panels heighten the transparency of its outer envelope. Circular glass lenses and translucent panels protect the bathers' privacy: they draw daylight deep into the plan while blocking views. Differences in level are resolved by half-levels and clear glass bridges that open views across the space. The materials are golden Bath stone, quarried at Limpley Stoke, concrete, stainless steel and high-performance glazing; the floors are naturally split Kashmir granite. The lighting, largely fibre optic, can be adjusted to set the atmosphere. Wide expanses of glass mirror the spa and the neighbouring buildings of Beau Street and Bilbury Lane, and the lighting and its rippling effects change the look of the New Royal Bath from day to night.

The project also restores old buildings. The Cross Bath, Grade I listed, rebuilt in 1783-84 by Thomas Baldwin and remodelled in 1789 by John Palmer, is an open-air bath standing on its own; Donald Insall Associates carried out its restoration. The Hot Bath, Grade II* listed, designed by John Wood the Younger in 1777 and adapted by G. P. Manners in 1831, had its central bath carefully roofed over; it is linked to the New Royal Bath. The main entrance and reception are at 7-7a Bath Street, a Grade I listed town house built in 1791 to designs by Thomas Baldwin.

The spa is in operation. Three springs in the centre of Bath, the King's Spring, the Hetling Spring and the Cross Spring, supply water that reaches the surface at 45 °C on average and cools to about 34 °C for bathing. It feeds all four baths: the Minerva Bath and the rooftop pool in the New Royal Bath, the Hot Bath and the Cross Bath.

The open-air pool looks out over the city and the surrounding hills, by day and by night. The Minerva Bath, the largest of the baths, stands out for its curves and grand columns. On the second floor, the Wellness Suite includes two steam rooms. The Hot Bath is used for treatments, among them Watsu, a water shiatsu; the separate Cross Bath has its own bathing sessions.

The spa is for visitors aged 16 and over, and proof of age is required on entry. The two-hour session, which gives access to the rooftop pool, the Minerva Bath and the Wellness Suite, with a towel and robe, can be booked online or by phone; booking ahead is strongly recommended, as a place on the day cannot be guaranteed.

The spa is open from 9 am to 9.30 pm (pools and Wellness Suite until 9 pm), except on Christmas Day, Boxing Day and New Year's Day.

The entrance is on Bath Street, in the centre of Bath, about 180 m from the towers of Bath Abbey.