25% glazing rule

[RandomUserName]

We have had plans drawn up for a 3.8 x 7 rear extension, meaning that we will be allowed approx 7.3m2 of glazing. however we want 6m sliding doors, which will be approx 12.5m2 glazing. Through reading, I thought that we'd be ok as we are removing quite a large amount of existing glazing (approx 9m2) from the rear of the house and that this could be added to our allowance. The guy who is doing our drawings has said that this is not the case though and we'll have to get triple glazed sliders, to conform with building regs. I've checked with the door manufacturers & this will add over £4k to the cost of the doors so would rather not go down that route. Just looking for some clarification, have I misunderstood how the rule works, attached is a pic of the plans & also the existing glazing to be removed. Thanks

 

[aveatry]

I had this rule malarkey but what effected it was that I not long before had a new condensing boiler and it was an extension with up to current regs insulation built in so although my plans were done as per the % the build reg guy told me I could have more --- so I did. But each council may be different.
So you may be allowed more glass but you have to make up for it somewhere else.
That's what peeves me off about these grand design houses - if you look all the glass is only on one side of the house and they never mention any glass rules and they just show you these wonderful homes.

 

[freddiemercurystwin]

Typically these issues are not hard to overcome, you just need an Energy Assessor to do some number crunching and typically add some additional insulation in the roof or walls. There's a Microsoft Excel spreadsheet Calculation knocking about on the web that used to be accepted by Local building Control, I'm not sure if it still is but it might be worth having a look for it and discussing with your BCO. Your designer sounds a bit amateurish, I've done highly glazed extensions a few times and never had to resort to triple glazing. Plenty of threads on the site eg: https://www.diynot.com/diy/threads/25-floor-area-rule.266263/

 

[Notch7]

We have had plans drawn up for a 3.8 x 7 rear extension, meaning that we will be allowed approx 7.3m2 of glazing. however we want 6m sliding doors, which will be approx 12.5m2 glazing. Through reading, I thought that we'd be ok as we are removing quite a large amount of existing glazing (approx 9m2) from the rear of the house and that this could be added to our allowance. The guy who is doing our drawings has said that this is not the case though and we'll have to get triple glazed sliders, to conform with building regs. I've checked with the door manufacturers & this will add over £4k to the cost of the doors so would rather not go down that route. Just looking for some clarification, have I misunderstood how the rule works, attached is a pic of the plans & also the existing glazing to be removed. Thanks

You don’t have to use elemental values or stick to 25% glazing.

You can get a SAPS calculation done, as Freddiemercurystwin says.

In simple terms you calculate the whole house energy use before the extension, then you do it for the whole house with all the proposed changes and compare the 2.

If you have had any work done recently…say this year, you would be allowed to include that as part of the main project, so if you’ve had a new boiler, had windows replaced, changed your light bulbs from old incandescent to LED, or insulated the loft, they can all count.

SAPs calc might cost you £250 or something