Swimming pool, Buckinghamshire

The site is large and accommodates a collection of outbuildings, many dating back to the late 19th century. But the estate no longer serves as a farm, and there is a lot to do to keep things running and, bit by bit, turn the site and its buildings into a modern home for a family with four children and several staff.

Paul+O describe themselves as the owner's in-house architects; they have been involved with the site for around five years, initially through a family connection. Some of the buildings lent themselves to conversion, or have even been added to in a traditional style. While this isn't typical work for Paul+O, they have built a charming brick greenhouse with a timber roof that blends seamlessly with the Victorian greenhouses on the site. An outbuilding has been refurbished as a two-bedroom home for the family's nanny, and the estate's gatehouse will soon be enlarged to accommodate resident staff. Other shed structures and farm buildings are still waiting to be reused or demolished, while various gardens, formal and informal, are taking shape.

When I tour the site, I get the feeling that this is a project close to the architects' heart, and that over the years they have gotten under the skin of this estate and learned how to add to its Victorian charm and complement its buildings' flaws.

The recently completed swimming pool is without doubt the jewel in the crown of the new work. It is a large building whose construction was only allowed because it replaced an equally substantial barn; building in the green belt is notoriously difficult, and much of the work on site has been through lengthy planning battles.

The swimming pool's exterior skilfully reconciles respectful contextualism with architectural ambition. The shape is distinctly barn-like, but has been distorted with an asymmetrical roof. This adds interest, particularly to the inside. The building is clad in a layer of dark red brick with some nice variations in colour; initially the architects championed a fair-faced concrete building, but the client's desire for a more literal relationship with the main house, a red brick building, won the argument. The Belgian brick is slimmer than the English standard and this takes away some of the heaviness of the massing. The pitched roof is finished in handmade Turkish clay tiles whose advertisement irregularity blends with the mostly handmade materials of the older buildings on site.

Once inside the swimming pool building, architect Paulo Marto slides open the large floor-to-ceiling windows, a Swiss product, and this dissolves the building's corners towards the garden. There are framed views of trees to either side, and glimpses of the house and the landscape beyond.

The interior is clearly designed in the modernist tradition, with clean, straight, undecorated surfaces, hidden services, and a minimum of materials; there is no more to it than white rendered walls, a dark stone floor and large windows with slim profiles, but all detailed and built with precision and care. Anticipating the noise that four children would be certain to make when enjoying the pool, the building has been fitted with an acoustic ceiling so there are no echoes or deafening splashing sounds. Beyond the pool and in the same building envelope is a gym, and above that, a play space with a window overlooking the pool. Externally the building serves as the fourth wall of an enclosed garden, and a low-level window in the pool building gives views of lavender bushes and the vegetable beds beyond.

It is maybe a little sad that a building which works so well in its local context and relates so successfully to the typically English arrangement of country house, outbuildings and formal gardens is made mostly from non-English materials. It would be good to see the English building industry facing up to the stiff competition of high quality and cost-efficient products from abroad. Nevertheless, this is a minor quibble, and does not detract from the building's success as a remarkable addition to this estate and its beautiful setting.