A dark walnut staircase merges with a kitchen counter and a bathtub inside this renovated apartment in Mayfair, London, by architecture studio KHBT (+ slideshow).

The combined elements create a dark wood strip that stretches through the apartment from the kitchen at one end to the bathroom at the other, negotiating split levels by staggering up and down.

"The existing flat already had a stair running from one room to the other due to head height restrictions at the main public staircase," KHBT partner Karsten Huneck told Dezeen. "The former design tried to hide this stair, but we saw it as the defining element of the flat which should be celebrated."

"This approach led to the concept of creating one ribbon which runs even further than the stairs require, incorporating the kitchen block as well as the bathtub," he added.

Describing the decision to use walnut, Huneck explained: "We wanted to create a precious element, using a timber that refers to the traditionally and luxuriously fitted flats in the surrounding area of Mayfair."

The architects kept the layout of the apartment as it was, but cleaned up the spaces with white walls, floors and ceilings that contrast with the dark wooden strip.

A glass screen is positioned in front of the bath and can be switched from transparent to translucent for privacy when residents are using it.

KHBT have offices in both London and Berlin and recently completed a timber-clad house extension in Offenbach, Germany.

See more apartment interiors on Dezeen »

Photography is by Johannes Marburg.

Here's a project description from KHBT:

No 1 Balfour Place - Residential Development in Mayfair

A rundown flat in prestigious Mayfair has been transformed into an inhabitable sculpture.

Formerly there were 2 separate flats which have been connected with a space defining spine. Due to head height restrictions of the communal staircase running across this spine various steps and level changes had to be overcome.

This led to the main concept, a meandering ribbon which becomes an inherent part of all main functions of the flat: Kitchen, Stair, Circulation and Bathroom.

The ribbon runs through a glazed separation between corridor and bathroom, a screen that is made of electrochromic glass which can be changed to be an opaque screen.

Made out of precious massive walnut timber it creates a fitting character to typical flats in the area and at the same time a contrasting element to the white flooring.

Above: floor plan

Client: Konstantin Mühling
Design Team: KHBT Karsten Huneck, Bernd Trümpler

Above: section