Why a basement room stays dark, and what the lightwell costs you
A basement gets its daylight from one direction only, and a conventional lightwell delivers that light sideways through a window at the bottom of a deep, narrow shaft. Basement walk-on glass turns the source upwards, so the room is lit from the sky rather than from a wall of brick a metre away.
Think about what a traditional lightwell actually does. The window at its base looks out at a retaining wall, often painted white to bounce what it can. Sky is only visible if you stand close to the glass and look up. The daylight that reaches the room arrives at a low angle, falls away within two or three metres of the window, and leaves the back half of the basement relying on lamps from October to March. On a deep Cambridge plot, where a basement under a Victorian or Edwardian house can run the full depth of the building, that back half is the part you most want to use.
The lightwell also takes something from the garden or front area above. An open well needs a railing or a grille around it, it collects leaves and litter, and it breaks up a small garden into a strip of paving on one side and a strip of lawn on the other. On the front of a terrace, the well and its railing can take most of the space between the bay and the pavement. That is the trade people are trying to undo when they ask about a basement skylight they can walk on: keep the light, get the ground back.
A flush pane of walk-on glass laid at ground level does both jobs. The garden or path runs straight across it, the railing can go, and the basement gets light from overhead, which travels much further into the plan than light arriving sideways through a vertical window. It is not the right answer for every basement, and the sections below set out where it works, what it involves underneath, and the regulations that decide whether an existing lightwell can simply be closed over. The main walk-on rooflight installation page covers the product family as a whole; this guide stays with the basement.
Three ways to use walk-on glass above a lower room
There are three common layouts, and which one suits a house depends on how deep the basement runs, where the existing lightwell sits, and whether the window at its base has another job to do.
The first is to cover the existing lightwell. The well stays as a void, the window at its base stays, and a pane of walk-on glass closes the top at ground level. The room gains a little overhead light through the well, the garden gains the area of the well, and the railing can come out. The light gain is modest, because the glass is still lighting a shaft rather than the room itself, but it is the least invasive option and the shaft keeps its drainage.
The second is a strip of glass along the house wall. Where the basement extends out under the garden, as many modern basement extensions do, a long narrow run of walk-on glass laid against the rear wall of the house lights the part of the room furthest from any window. Light washes down the internal face of that wall, which reads as a bright surface from across the room and makes a low ceiling feel higher.
The third is a separate panel set out in the garden, directly over the deepest part of the basement. This is the option that transforms the back of a long plan, because it puts daylight where no window could reach. It asks the most of the structure, since the opening sits in the middle of a slab carrying soil, paving or planting.
| Layout | Light below | Garden gained | Works involved |
|---|---|---|---|
| Cover the lightwell | Modest, into the shaft | The whole well | Lightest |
| Strip by the wall | Washes the back wall | Railing removed | Moderate |
| Panel in the garden | Deep into the plan | None lost | Heaviest |
Some houses combine two of these: the front lightwell covered to tidy the front area, and a panel at the rear to light the back of the room. For gardens where the glass will sit among planting and seating, our guide to walk-on glass in courtyards and gardens covers the landscape side in more detail.
Where the light lands in the basement
Horizontal glass at ground level sees the whole sky above it, and that changes the character of basement daylight more than any increase in glass area would. The question is less how much light comes in and more where it falls and when.
On a clear day the sun throws a sharp shape of the pane onto the basement floor or wall, and that shape moves. In a Cambridge garden with no hills and few tall buildings to shade it, the glass is exposed to the sun from mid-morning to late afternoon in summer, and the bright shape can travel several metres across the room in the course of a day. In winter the sun stays low, so the direct beam comes in steeply angled and lands on the walls rather than the floor, and for much of the day the glass delivers soft sky light instead.
That makes position the first decision. A panel on the north side of a house, in the building's shadow for much of the day, gives steady, even light with little direct sun, which suits a home office, a media room or a studio. A panel out in an open south-facing garden gives far more direct sun, which is welcome in a playroom or a kitchen in winter and can be too much over a desk or a screen in June. The light plan we write with every quote marks where the sun patch falls at a few times of day and year, so you can see whether it lands on the sofa, the stair or the bare wall before any ground is dug.
The finish on the glass changes the result as much as the position does. A clear pane gives a hard-edged pool of sunlight and a view of the sky; a sandblasted or fritted top surface scatters the light into a softer glow across the room and hides the view up from the basement. The detail on this sits in our guide to glare and pools of sunlight below walk-on glass.
Keeping the garden usable over the glass
The appeal of glass at ground level is that it disappears into the garden, and that only works if the surface can be treated like the paving around it. That depends on grip, privacy and what the glass is expected to carry.
Grip matters more here than on a roof terrace, because a garden path gets walked on in wet shoes, with leaves on it and sometimes with frost. The top pane of walk-on glass is normally given a textured finish for this reason, and the maker should be able to say how it performs when wet. Our guide to slip resistance of walk-on glass explains the finishes and how they are tested.
Privacy works in both directions. From the garden, a clear pane shows the room below, and from the basement a clear pane shows whoever is standing on it. Over a bedroom or a bathroom, an obscured finish is the usual choice. A separate guide to privacy finishes compares the options and what each does to the light.
Then there is load. A garden gets used in ways a flat roof never is: a ladder set up to cut a hedge, a heavy barbecue wheeled across, a stack of paving slabs put down during other work, a large pot of soil left in the sunniest spot. Walk-on rooflights are designed for pedestrian use, and each product carries a declared rating for a spread load across the pane and for a concentrated load on a small area. What any particular unit is good for depends entirely on the maker's declared rating for that product and size. Check that rating against how the garden will actually be used, and do not assume a unit rated for foot traffic will take a ride-on mower, a vehicle or a heavy planter. The walk-on rooflight load ratings guide explains what the published figures mean and which older ratings to avoid.
What sits underneath: slab, upstand and waterproofing
Over a basement, the walk-on glass is only the visible part. Underneath it is a structural slab, an opening formed in that slab, a kerb or frame the glass bears on, and a waterproofing system that has to stay continuous around the whole edge.
Most basement roofs under a garden are reinforced concrete slabs, sometimes with steel beams, carrying a build-up of waterproofing, insulation, drainage layers and then soil or paving. The opening for the glass has to be formed in that slab with the edges trimmed to take the load that the removed concrete used to carry, and the glass needs a level, square seat with continuous support along its edges. None of that is a detail to guess at. The size and reinforcement of the opening, and the design of any trimming beams or kerbs, are for a structural engineer to set, and the glass must be ordered to the engineer's opening, not the other way round. Our guide to the structural opening for walk-on glass goes through what the engineer specifies, and the short answer on whether walk-on glass needs a structural engineer covers when one is required.
Waterproofing is where basement work differs most from a roof terrace. A basement is protected by a designed system, often described using the three types in the British Standard for below-ground waterproofing, BS 8102: a barrier applied to the structure, a structure built to resist water itself, or a drained cavity that collects water and pumps it away. Whichever system the basement has, the glass opening is a break in it, and the new edge must be tied into that system so that water is kept out at the kerb. The basement designer or the waterproofing specialist who installed the original system should be involved in how that junction is detailed, and it may affect any guarantee that system carries.
Drainage at ground level matters too. A flush pane in a garden sits at the lowest point unless the surrounding surface falls away from it, and soil and mulch wash onto glass in heavy rain. A channel drain around the edge, or falls in the paving that take water away, keeps the glass clear. The detail on this is in drainage around flush walk-on rooflights.
Covering a lightwell: escape, air and guarding
Closing over an existing lightwell looks like a simple improvement, but the lightwell may be doing a job the Building Regulations require, and that has to be checked before it is covered.
The first question is escape in a fire. In a dwelling, a habitable room in a basement generally needs either a way out to a protected stair or an escape window, and the window at the bottom of a lightwell is often the basement bedroom's escape window, with the well itself as the route to the garden. A fixed pane of glass over the top would close that route. There are ways round it, such as an opening walk-on unit, a different escape arrangement, or leaving part of the well open, but which of them is acceptable for a particular house is a Building Control question, and it should be settled before anything is ordered. Opening walk-on units exist, but they are more limited in size and finish than fixed ones.
The second is air. If the basement window opens for ventilation, covering the well above it cuts off the fresh air it was supplying, and the room may then need another route for background and purge ventilation. The third is the reverse of a problem: an open lightwell is a drop that needs guarding, and covering it with glass rated for foot traffic removes the fall risk and the need for the railing, provided the glass is designed for that use.
None of this rules out covering a lightwell. It means the lightwell's current role is part of the survey, and the plan for the room has to show where escape and ventilation will come from once the glass is in.
Planning and Building Control in and around Cambridge
Whether a basement walk-on rooflight needs planning permission depends on where the house is, which part of the plot the glass sits in, and whether the basement itself already has consent. Building Control is a separate question and applies far more often.
Flat glass laid flush in a rear garden changes little in how a house looks from the street, and on many houses that kind of work falls within permitted development or is covered by the permission for the basement. The front area of a terrace is different: removing railings and a lightwell at the front changes the street scene, and in the city's conservation areas, which take in large parts of central Cambridge including many of the terraced streets around Mill Road, Newnham and De Freville, the planning officers at Greater Cambridge Shared Planning may take an interest. Flats and maisonettes have no permitted development rights for this kind of work, and a listed building needs listed building consent for any alteration that affects its character, including below ground. The answer on whether a walk-on rooflight needs planning permission covers the general position. Check with the local planning authority before committing if the house is listed, is a flat, or sits in a conservation area and the work is at the front.
Building Control is more consistently involved. Cutting an opening in a structural slab, changing a means of escape and altering ventilation are all matters the Building Regulations cover, and walk-on glass has to meet the requirements for structure, for safety glazing and for heat loss. Where the work is notifiable, we handle the Building Control notification, and we install to current Building Regulations. The Building Regulations for walk-on rooflights guide sets out which parts apply.
Replacing old pavement lights and gratings
Some older Cambridge houses already have glass over a cellar or a front area, in the form of a pavement light or an iron grating, and replacing it with a modern walk-on pane is often the simplest way to bring more light down.
Traditional pavement lights are small glass lenses or prisms set into a concrete or cast-iron grid. They were designed to get some light into a coal cellar, not to light a room, and after decades underfoot the lenses are often cloudy or cracked and the grid corroded. Gratings let in more light and more water. A single insulated walk-on pane in the same opening gives clearer light, a sealed, insulated top to what may now be a habitable room, and a surface that is easier to keep clean. The answer on whether a pavement light is the same as a walk-on rooflight compares the two.
Two cautions apply. If the old light sits in a public footway rather than inside the property boundary, it is a matter for the highway authority, and different loads and permissions apply. And an existing opening sized for a grid of lenses will not necessarily suit a single pane; the edge support, the waterproofing and the depth of an insulated unit all need checking, so the engineer and the maker's details for the chosen product decide whether the existing opening can be reused or needs to be re-formed.
How a basement walk-on job runs
A basement job starts with a survey of the room below as much as the ground above, because the light plan is about the basement, and the structure and waterproofing drawings decide what is possible.
- Light survey. We look at the basement, how it is used, where the existing lightwell and windows are, and which way the garden faces, then mark where overhead glass would put the light at different times of year.
- Structure and waterproofing. The structural engineer designs the opening and its support, and the basement waterproofing designer confirms how the new edge ties into the existing system.
- Specification. The glass is chosen for its declared load rating, grip, privacy finish and thermal performance, and ordered to the engineer's opening size.
- Regulations. Planning is checked where it applies, and Building Control is notified where the work is notifiable, including any change to escape or ventilation.
- Installation. The opening is formed, the kerb and waterproofing completed, the glass set and sealed, and the surrounding paving or planting made good to the finished level.
Every quote comes with a written light plan showing the position, size, glass and finish, and the reason for each, and the installation work carries our 10-year workmanship guarantee. To see how the light would fall in your basement, book a light survey. Typical cost ranges for walk-on glazing are on our costs page.
Questions people ask
Will walk-on glass make my basement warmer or colder?
An insulated walk-on unit loses more heat than the insulated slab it replaces, so the basement will lose a little more heat through that area in winter and gain some from the sun in summer. The thermal rating of the chosen unit, its glass and the size of the opening decide how much, which is why the specification is set against how the room will be used.
Can I put a table or planters on the glass?
That depends on the declared rating of the specific product and on the structural engineer's design for the opening. Heavy pots and table legs concentrate load on small areas, so check both before treating the glass as part of the patio.
Does the basement need to be finished before the glass goes in?
Not always. On a new basement, the opening is best formed when the slab is designed and cast, with the glass fitted near the end of the work. On an existing basement, the opening is cut and formed from above, and the room below needs protecting from dust and water while the slab is open.
