We want to increase the size of our windows. Can it be done?

Property Clinic: Will planning permission be required?

We would like to replace our uPVC windows but want to increase the size of the window frames to let more light into the house. How do we assess if this is structurally possible without significant expense? Will planning permission be required?

The arrangement and size of windows or openings on an elevation or wall is referred to as fenestration. In their desire to encourage vernacular design and traditional looking houses, local authorities have encouraged the use of windows with a vertical emphasis. This results in windows that are much narrower than the large horizontal windows featured in familiar bungalows. With narrow windows comes reduced light and I can attest to this as many building surveys now reveal issues with daylighting and undersized windows.

A general rule of thumb when designing is that the window size should be at least 20 per cent of the floor area of the room it serves. This ensures adequate daylighting.

Widening of the window opening width generally cannot be undertaken without major work. The lintels above the window would need to be replaced if the openings are to be widened. This is because the supporting lintel above the window usually only extends about 100mm beyond the opening. This is the minimum needed to disperse the weight of the wall above. Therefore, any new lintel will need to extend at least 100mm beyond the sides of the enlarged opening.


Similarly, the window cill would need to be removed and replaced with a larger cill meaning an amount of demolition work.

Careful builder

I suggest that you review your windows to establish if they can be enlarged vertically rather than horizontally. This means that the lintel above the window can be left undisturbed. The window width can remain but the cill height is lowered. If you have decorative or expensive window cills these could be reused. You will need a careful builder. The new window openings will also retain that ever important vertical emphasis.

The fenestration on your house would have been approved through planning permission. It may also match other adjacent properties if your house forms part of a larger development. Alterations to fenestration affects a building’s character and would therefore require planning permission. All replacement windows would need to meet building regulations in terms of the requirement to provide means of escape, use of toughened glass, prevention of heat loss and the like.

Daylighting is vitally important in homes and adds to one’s enjoyment and general wellbeing. The need to provide homes with traditional proportions should not have a detrimental effect on this. Before beginning to correct the errors of the past make sure you have fully evaluated all possible remedies.

When reviewing your proposed alterations seek the advice of your local chartered building surveyor. They will be able to assess any potential obstacles and perhaps suggest more manageable solutions such as the use of roof lights to supplement daylighting. They will also be able to prepare the planning application on your behalf.

Best to think your plans through carefully before reaching for the demolition hammer. – Noel Larkin

Noel Larkin is a Chartered Building Surveyor and member of the Society of Chartered Surveyors Ireland. scsi.ie