Why a bigger window is a different job from a swap
A like-for-like replacement drops a new frame into a hole that already exists; a larger window means cutting a new hole around the old one and rebuilding everything that met it.
When a VELUX roof window is replaced at the same size, the timber opening, the trimmers and the internal lining can all stay put. The old sash and frame come out, a new frame and flashing go in, and the plaster reveals are left almost untouched. Our answer on whether a new VELUX will fit your old opening covers that route in detail.
Going bigger removes those shortcuts. Every edge of the opening moves, so the tiles or slates around it are stripped back, the battens and underlay are cut and re-dressed, and the inside lining is taken down to the rafters. In practice the job looks much more like fitting a VELUX where there was none than like a swap, and it is priced and planned that way. The step-by-step sequence is set out in the VELUX window installation process.
Taller or wider: the rafters decide
Making a window taller usually leaves the rafters alone; making it wider almost always means cutting one or more of them.
VELUX sizes are coded by width and height. The letter gives the width (C is 550mm, F 660mm, M 780mm, P 940mm, S 1140mm, U 1340mm) and the number gives the height (04 is 980mm, 06 is 1180mm, 08 is 1400mm, 10 is 1600mm). Our guide to VELUX window sizes and codes explains how to read the code on the data plate of the window you have now.
Rafters in a Victorian terrace or a 1930s semi are typically set at around 400mm centres, sometimes wider, so even a small C-width window may already sit across a cut rafter. Height runs up the slope, along the rafters. Going from an MK04 to an MK08 adds 420mm of height but stays in the same bay: the head and sill trimmers simply move further apart and the rafters either side are untouched. Width runs across the slope, so going from an MK06 to an SK06 adds 360mm and will usually take out another rafter.
| Change | Example | What happens to the roof |
|---|---|---|
| Taller, same width | MK04 to MK08 | Trimmers moved; side rafters stay |
| Wider, same height | MK06 to SK06 | Extra rafter cut; sides doubled |
| Taller and wider | CK02 to PK08 | Full new trimmed opening |
Trimming the opening
A cut rafter no longer carries its share of the roof, so that load has to be picked up by timbers around the new opening.
The standard method is to double the full-length rafters on each side of the window (the trimming rafters), then fix horizontal trimmers across the head and the sill to catch the ends of the cut rafters. The frame then sits in that timber box with the clearance set out in the VELUX instructions. For a single cut rafter this is routine carpentry. Where two or more rafters come out, or where the roof already carries a purlin, a water tank or a heavy covering such as clay tiles or slate, Building Control may ask for the timber sizes to be checked by calculation, and a structural engineer may be needed.
Some things cap how big a window can go. A purlin running across the slope, a chimney breast, a valley or a hip set hard limits on position. On a terrace, the Building Regulations fire guidance generally keeps rooflights out of a strip of roof about 1500mm either side of the party wall, which on a narrow Romsey or Mill Road house can leave less width than the owner hoped for.
Flashing, lining and the parts that do not carry over
A larger window needs its own flashing, insulation collar and blind, and the inside reveals are formed again from scratch.
The flashing is sized to the frame, so the old one cannot be reused. The right type depends on the roof covering: a profiled tile, a flat tile or a slate each takes a different kit, as VELUX flashing types explained sets out. Around the new frame we fit an insulation collar and vapour barrier so the bigger opening does not become a bigger cold spot; our guide to insulation collars and installation kits shows why this matters for condensation.
Inside, the lining is rebuilt with the head reveal set level and the sill reveal set plumb, the shape VELUX recommends because it spreads the light down into the room rather than onto the ceiling. Any existing blind will be the wrong size, so a new one is part of the job.
Building Control and planning
Enlarging a roof window alters the structure and the thermal envelope, so it is normally notifiable under the Building Regulations; planning is a separate question.
Cutting rafters brings in Part A (structure), and a new, larger window has to meet Part L for thermal performance. Near a boundary, Part B (fire) applies as well. A like-for-like swap is treated differently from a new or enlarged opening, so it is worth checking before the roof is opened. Where the work is notifiable, we handle the Building Control notification.
On planning, a rooflight on a house can usually be installed under permitted development if it projects no more than 150mm from the roof plane and does not rise above the ridge. In a conservation area, on a listed building or where an Article 4 direction applies, Greater Cambridge Shared Planning or the local district council may take a different view, and a larger opening on a street-facing slope draws more attention than a small one. Read do you need planning permission for VELUX windows before settling on a size.
Will more glass give you the light you want?
A bigger window lets in more light, but where that light lands, and how much heat comes with it, depends on the slope it faces.
On a south-facing slope, doubling the glass area also roughly doubles the direct sun and the summer heat pouring into a loft room, and a large pane over a desk or a bed can make glare harder to live with. On a north slope a larger window is usually pure gain: steady, cool light with little glare. East and west slopes sit between the two, with low morning or evening sun reaching deep into the room. Moving the window higher up the slope, or choosing solar control glass, can matter as much as the extra size. Our page on how we plan the light explains the thinking, and every written quote carries a light plan that sets out the size, position and glass for your room with the reason for each.
Questions people also ask
How long does it take to fit a bigger VELUX window?
Plan on the time of a new installation rather than a swap: one to two days, with the lining and making good inside taking a good share of it. The detail is in how long a VELUX installation takes.
Can a bigger window be fitted from inside the loft?
The frame can often be handled from inside, but enlarging the opening means stripping and relaying tiles around it, so roof access by scaffold or tower is normally needed.
Does a bigger window cost much more than a same-size swap?
Yes, because the carpentry, tiling, flashing, lining and Building Control are all extra work. The factors are laid out in what affects the cost of fitting a VELUX window, and a survey sets the real figure.
Who can advise on the right size for my room?
That is what our survey is for. We look at the rafters, the slope and the room, then recommend a size and position. See VELUX window installation in Cambridge, or book a light survey.
