A bay window is one of the few window jobs that planning law treats as an extension rather than a window, because it projects out from the wall. That single fact decides most cases, and it is why replacing an existing bay is usually straightforward while adding a brand new one to the front of the house often is not. Here is how it works in England, with the local wrinkles we see around Dunstable and Central Bedfordshire.
If you already have a bay and you are swapping it for a new one of the same size and projection, that counts as an alteration rather than an extension. For a house, this normally falls under permitted development and you will not need a planning application, even if you are moving from timber to uPVC or aluminium, and even in a conservation area.
Two things change that answer. Flats and maisonettes have no permitted development rights at all, so any external alteration needs consent from the council. Listed buildings need listed building consent for window replacement full stop, regardless of what the windows are made of or how poor their condition is.
Permitted development rules do not allow any extension to project beyond a wall that forms the principal elevation and fronts a highway. A bay window projects by definition, typically 300mm to 600mm, so adding one to the front of a house facing the road will normally need a householder planning application. The same applies to a side elevation that fronts a highway, which catches a lot of corner plots on estates around Dunstable and Houghton Regis.
Bays to the rear, or to a side that does not face a road, are more likely to be permitted development, but they still eat into your extension allowances and must sit within the height and depth limits. A bay at first floor level is a different proposition again, because it is no longer single storey and the rules tighten considerably.
Two local points worth checking. Newer estates often have planning conditions attached to the original consent that strip out permitted development rights entirely, so the general rules do not help you. And an Article 4 direction, sometimes used in conservation areas including parts of central Dunstable, can remove the right to alter windows at all.
Planning permission and Building Regulations are separate systems, and a bay window engages both. Any replacement window has to meet current thermal standards, which in practice means a Window Energy Rating of band B or better, or a whole window U-value of 1.4 W/m²K or lower. Glass in and near a bay usually falls into a critical location, so toughened or laminated safety glass is required, and background ventilation has to be maintained where the old window had it.
The structural side is where bays catch people out. If there is a roof, a wall or a bedroom sitting on top of the bay, that load has to be supported properly during and after the work, and that is not something a standard FENSA or CERTASS self certification covers on its own. Widening an opening or altering the structure means notifying Building Control, and on a bay carrying anything above, expect a beam or lintel specification from a structural engineer.
Start with the council's planning portal to see whether your address sits in a conservation area and whether the original consent for your house carries conditions. Central Bedfordshire Council also offers a pre-application service if the case is borderline.
If you conclude the work is permitted development, it is worth spending the fee on a Lawful Development Certificate. It costs roughly half a householder application, both take around eight weeks to determine, and the certificate is the document a buyer's solicitor will ask for in ten years' time. A householder application in England currently sits a little over £250, though fees change, so check the current figure when you apply.
Get the planning position settled before anything is measured and manufactured. Bays are made to order and non returnable, and no installer can unwind a bay that the council later asks you to remove.
For planning purposes, yes. Because it projects beyond the wall line it is assessed under the extension rules rather than the window rules, which is why a front facing bay so often needs an application.
Only if the extra projection is itself acceptable, which on a front elevation facing a road normally means applying for permission. Increasing the projection also usually means new foundations and a revised structural design rather than a straight swap.
Most window installers, including us, will tell you where you stand and flag anything that looks like it needs consent, but the application itself is made by the homeowner or an agent acting for you. It is a straightforward form for a simple bay, and an architectural technician can prepare the drawings if you would rather not.
Tell us about your job and we will get back to you the same day to arrange a free, no-obligation survey.