This house in Belgium has been overhauled for a couple of art and book collectors to include a wood-lined living space and plenty of cosy reading nooks.

Belgian architect Rob Mols designed a one-storey extension for the semi-detached house in the city of Leuven, adding a gym, an open-plan living space and a master bedroom. Antwerp-based Studio K  oversaw the interior fit-out of both the extension and original house.

The timber-framed extension is located three metres below street level and features a sawtooth roof with clerestory glazing that brings in light from above.

Inside, the wooden structure is left exposed and glazed walls offer plentiful views of the garden. They also frame views of a courtyard that sits between the gym and the living room.

Mols' extension forms part of a wider renovation of the existing property, which was reconfigured to create different reading nooks and storage options.

Studio K's interior design contrasts dark intimate spaces with lighter and more open areas.

A reading space is located next to one of the large windows in the lounge, which has a shelf integrated into its frame. In the kitchen, a partition slides open to reveal dark kitchen cabinets and more space to store books.

"The customer had a specific demand to provide plenty of space for storage purposes and exhibit books and paintings, together with creating different reading and leisure areas each with their own function and atmosphere," said Studio K.

A library lined with dark wood now occupies the former entrance hall of the existing property. It is connected to the garden-level extension by a staircase with cantilevered oak treads and a white-painted frame.

"The library is located next to the entrance area," said the designers. "The dark-stained oak floor and dark-stained oak veneer bookcases causes an intimate atmosphere."

A formal dining area is located next to this reading space, which features glass doors that lead to a terrace on the roof of the extension.

A corner for displaying items collected by the couple on holiday, a second bathroom and office occupy the first floor. A new staircase leads up to the attic where there is a guest bedroom and a movie room.

Studio K finished the rest of the extension simply with polished concrete floors and white-painted walls that complement the light-toned wood.

Beside the kitchen a sliding door opens to the new master bedroom, which has one wall painted in dark turquoise.

A pair of bathroom sinks contained in a freestanding island containing separate the bedroom from the en-suite bathroom, which is covered with dark green tiles.

Other recent residential extensions in Belgium include a pair of gabled structures added to a farmhouse by Atelier Tom Vanhee and a terraced house by Edouard Brunet and François Martens with a glazed kitchen protruding from its top floor.

Photography is by David Dumon.