The Finns know how to do winter. This open-plan penthouse in Helsinki is warm and inviting while still being bright and airy. The alder-wood sauna is a particularly welcome treat on the coldest days

Few cities experience such extreme seasons as Helsinki. after the giddiness of summer when the sun barely dips below the horizon and every night is a celebration of life outdoors, winter descends like a bleak punishment for having had it so good. Anyone brought up reading about Tove Jansson's Moomin will remember the foreboding of winter with the blocks of black-and-white etchings, which meant snow, darkness and hibernation.

The top of a five-storey 1924 apartment building designed by renowned Finnish architect Lars Sonck is the perfect place to enjoy winter. Coming in from the cold, you enter an original wood-panelled Kone lift – complete with foldaway bench, concertina grille and brass Kone plaque – which shuttles you smoothly to the top of the building.

The 180 sq m apartment is not a Sonck originally designed it. The current owner ripped out the six rooms and servant quarters in 2005, transforming it into an open-plan penthouse. Thanks to Sonck's experimental structure – in which the roof is supported not on walls but on iron-and-concrete columns – it is a cavernous space punctuated only by a couple of columns, a kitchen unit and a bookcase.

This clean, white setting is a blank canvas for the owner's extraordinary collection of Finnish furniture, glassware and ceramics. It's like and ode to Finnish design, peppered only with a few foreign objects: a low gry-wool Zanotta sofa, a couple of Le Corbusier club chairs and, in the bedroom, a late 19th-century Russian glass chandelier.

The owner describes himself as a minimalist who hates handles. He marvels over the seamless edge of the Corian kitchen sink and bathroom basin. The floor-to-ceiling cupboards reveal yet more Wirkkala glassware; this is no place for curious toddlers. The lights and electronics are all recessed into the walls to keep the lines pure and a caramel-coloured Canadian oak floor throughout softens the effect of the white and gives the apartment and earthy warmth.

From the balconied bathroom it is just a short hop to the built-in alder-wood sauna. Life up here is indulgent, surrounded by birds, books and beautiful furniture. When the clouds roll in and the corner fire is lit it quickly becomes snug, though never oppressive or stifling.

When the sun is shining outside the whole apartment, with the white walls and all that brass and glass, glows.