The role of natural light in design

In our projects, natural light is one of the first questions we address, long before finishes. It shapes the comfort of a living room, the value of a flat, and even part of the energy bill. Here is how we work with it day to day at STARC and Optibim, alongside the Brussels and Walloon regulations we apply.

What the regulations say, and why it is only a minimum

In Brussels, the Brussels Regional Planning Regulation (RRU, in Dutch GSV), Title II, sets the basic rule for housing. Habitable rooms, excluding kitchens, must be naturally lit. The net light-admitting surface must be at least 1/5th of the floor area. For habitable rooms whose light-admitting surface lies within a roof slope, this net light-admitting surface is set at a minimum of 1/12th of the floor area. The text also adds a view requirement: every dwelling must have at least one window allowing direct, horizontal views to the outside, unobstructed over at least 3 m.

In Wallonia, the reference is found in the habitability criteria of the Walloon Government Decree of 30 August 2007. SPW Logement, the Walloon public housing authority, summarises them as follows: for dwellings created after 1 January 2008, 1/12th of the usable floor area in vertical glazing and 1/14th in roof glazing. For older dwellings, the thresholds are 1/14th and 1/16th. As an example, SPW cites a living room with a usable floor area of 25 m², which would require 2.1 m² of glazing if created after 2008.

These figures remain minimum habitability thresholds. The Sustainable Building Guide published by Brussels Environment puts it plainly: meeting these criteria in no way guarantees that an adequate amount of natural light reaches the dwelling. A 1/5th glazed area facing a narrow courtyard on the ground floor does not produce the same effect it would on the fourth floor.

To go further, we rely on the European standard NBN EN 17037. According to Buildwise, the daylight criteria cover daylight provision, view out, direct sunlight and glare risk. For each criterion, recommended values are given on a scale of three performance levels.

Worth noting for Brussels: the reform of the RRU, known as Good Living, has been in preparation for several years. According to a legal analysis of the draft, the 1/5th rule was retained, but the natural light must now come from "one opening" rather than several. We therefore always check which version is in force at the time the planning application is lodged.

In our practice, light is designed from the very first sketch

The most important choice is made early, at the site layout stage. Before a single window is drawn, the team asks itself four questions:

• the orientation of living spaces: the living room facing south or west, bedrooms facing east, service rooms facing north;
• the depth of rooms: beyond 6 or 7 m from the façade, the back of a living room remains dark, whatever the size of the openings;
• the height of the lintels: for the same glazed area, a high window lights a room more deeply;
• obstructions: neighbouring buildings, trees, terrain, roof overhangs.

The Sustainable Building Guide gives a useful design benchmark: the net light-admitting surface located more than 1 m above floor level should exceed 12% of the net floor area of the room. Glazing that runs down to the floor gives an impression of openness, but adds little useful light to the back of the room.

Our tools, from survey to simulation

In renovation projects, everything starts with a reliable survey. With BimBot, our surveying brand, we scan the existing building in 3D and supplement this with drone footage and 360° photographs. This gives us the actual wall thickness, sill heights, party walls and surrounding obstructions, precisely the data that determine the real light levels in a room.

From this base, we build a SketchUp or BIM model and carry out several checks:

• a sunlight study at the solstices and equinoxes, to see when the sun actually reaches the living room or the terrace;
• a room-by-room compliance check (1/5th in Brussels, 1/12th in Wallonia), set out in a table attached to the planning file;
• for more ambitious projects (apartment buildings, office-to-residential conversions), a daylight simulation based on NBN EN 17037.

Office-to-residential conversions illustrate the value of this approach well. Office floor plates are often 15 to 18 m deep. The simulation quickly shows which zones can accommodate living spaces and which zones should house storage, bathrooms or a patio carved into the building's volume.

Light and energy, striking the balance with Optibim

More glazing brings more light, but also more risk of overheating in summer and heat loss in winter. This is where Optibim's EPB (energy performance) team steps in. Two indicators guide the choice of glazing:

• light transmittance, the proportion of luminous flux passing through the glazing within the visible spectrum;
• the solar factor g, the proportion of solar energy passing through the wall or glazing relative to the incident energy.

In practice, triple glazing lets through slightly less light than high-performance double glazing. Solar control glass reduces heat gain, but can darken a room and alter its colours. For a large south- or west-facing opening, we often prefer clear glazing combined with external solar shading (louvres, roof overhang, screens). This retains light in winter while blocking direct sun in summer. The EPB calculation then confirms that the overheating indicator stays within acceptable limits.

Practical solutions for renovations and farmhouses

Brussels terraced houses and farmhouses in Walloon Brabant or Namur province present the same challenge: thick walls, narrow openings and deep rooms. These are the solutions we use most often:

• opening up the rear façade, which is usually less constrained from a heritage standpoint than the street façade;
• creating roof windows or a lantern above a staircase, lighting two or three storeys at once;
• reusing existing large openings, such as barn or stable doors, while preserving the bluestone surrounds;
• installing glazed partitions or fanlights between rooms to bring in secondary light;
• splaying the internal reveals of thick walls, which noticeably diffuses the light;
• choosing light colours for ceilings and back walls, surfaces that reflect a good deal of light.

Three recurring pitfalls:

• altering a façade without checking whether planning permission is required under the CoDT (Walloon land-use code) or the CoBAT (Brussels land-use code);
• adding too many roof windows without solar shading, which causes the attic to overheat as early as May;
• overlooking that, in Wallonia, SPW Logement specifies that indirect light only counts if the adjoining room itself has sufficient direct natural light.

What we take from this

Natural light is not a finishing touch added at the end of a project. It follows from orientation, depth, opening height, glazing and solar shading. These choices are made together, within the same practice, by architects, engineers and EPB experts. That is precisely why STARC and Optibim work under one roof.

Are you planning a new build, a renovation or the conversion of a building, and wondering how to bring in more light? Contact STARC for architecture, or Optibim for EPB studies and BimBot surveys. We will analyse your building or site and propose concrete solutions in line with your region's regulations.

Frédéric Odent