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Exposed structure and plaster mark Catja de Haas’s basement office conversion

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A London terraced house’s basement has been remodelled as an office for the architect’s practice

As the basement floor had its own existing entrance, the internal stair was able to be removed in order to create more space, while one of the spine walls has been opened up, leaving the structure exposed. This open studwork wall divides a library area from a main workspace. There is small kitchen on one flanking wall with a new WC and shower leading off this.

The exposed structure is designed to create transparency, opening up a visible sequence of spaces and allowing light to penetrate. By leaving the open studwork partition in place where the spine wall had been, there was no need for large structural interventions or work on the foundations. 



The open office layout together ends in a glass-covered lightwell, which creates a seating alcove below the garden. 

Where possible, existing elements such as the bathroom floor and windows have been reused or kept. The original plaster has also been left exposed and blue 15x15 cm floor tiles run throughout, giving scale to the spaces.



Architect’s view

The project was supposed to be a simple refiguring of an existing floorplan to allow for an architects’ office that could eventually be converted into a spacious one-bedroom flat.

When damp testing was carried out before the start of the building works, the damp proofing expert discovered that damp had penetrated all walls in the basement and they had to be stripped. After stripping the walls and removing the stairs, it revealed the spine walls had to be replaced.

To avoid the placements of a complicated structure or extra loads on the foundation, we decided to rebuild the spine wall but leave the studwork open to create a semi-permeable wall. The wall divides the area into a library and a workspace with a small kitchen tucked in the corner. A shower room and toilet are also added so the basement space can also easily be converted into a one-bedroom apartment. 

A lightwell at the end has been created by internalising the previously outside space. The enclosed lightwell and window create a seating alcove with views over the garden. As part of the façade is still there, the spaces are marked by a series of semi-open walls.



The biggest difficulty was to balance the need to make the house’s energy performance better while retaining as many existing features as possible. We managed this process as follows: all facades have been insulated with 10mm aerogel; cold bridges have been avoided in the detailing; and the existing windows have been taped. To avoid the need to install a ventilation system, the natural vent from the chimney flue has been maintained. The brushes and insulation in the existing windows, which were double-glazed, have been replaced, but the double-glazed windows themselves have not been replaced.

The choice to keep the original plaster visible, the small coloured floor tiles and the stained wood of the stud wall, together with the layering of spaces that culminates in the lightwell, give the space a calm appearance.

The original tiles in the toilet and sanitary ware were all saved and reused.

The basement has developed and changed in use over the last 20 years in a house for a family of five. In the first years it was used as a play area and study for the children, and a room for an au pair when both parents were working outside of the house. When the children became teenagers it became a place for ‘gatherings’ and PlayStation.

The alcove was originally clad with corrugated plastic and formed a play area: a kitchen, a puppet show. The original façade is still there, its retention also preventing unnecessary loads on the foundation and creating a semi-separated space.

Catja de Haas

Office axonometric

Office axonometric

Project data

Start on site August 2019
Completion December 2019
Gross internal floor area 63m²
Form of contract or procurement route Homeowner’s contract
Construction cost Undisclosed
Architect Catja de Haas Architects
Executive architect Catja de Haas Architects
Client Catja de Haas Architects
Structural engineer Foster Structures
Damp proofing consultant/subcontractor Preservation Treatments
Project manager Catja de Haas/ Sophie Walker
Approved building inspector Integral Building Control Solutions Ltd
Main contractor Red Sky Property
CAD software used MicroStation

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