25% Floor Area Rule...?

Hi guys,

My project is a single store rear extension 3m x 5.7m (externally) made up of 100+100+100 wall costuction with the following windows/doors ;

window 1200x1200mm
tri-fold doors 2400x2100mm
2 x roof lights 1340x980mm

However, after coming across the following thread (specifically the 25% rule part), I am now concerned that the BC department wont accept my design.
//www.diynot.com/forums/viewtopic.php?p=1262947#1262947


My actual question is, what is defined as the 'floor area' ?

floor area of the extension (3x5.7m)

OR

the above + floor area of existing kitchen and lounge (as it will be open-plan)

Many thanks.

Floor area will be the internal new floor area size you have created by the new extension, then 25% of that area and add the m2 of any existing openings that will be covered by the new extension, this is your allowed glazed area. How far out are you ?
Would you be under Rushcliffe ?
Edited as I got it round me arse as Nickjb says bellow

AIUI

the maximum area of windows and doors and roof lights is calculated from:
Maximum area = 0.25AF + Aold
where AF is the floor area of the extension and Aold is the area of any windows or doors that no longer exist or are no longer exposed as a result of the new extension.

What is the area of the windows and doors you are removing?

I read somewhere that you might be able to argue that adding more windows/roof lights will reduce your energy consumption as you will need less lighting. Not sure if that is the official line though.

EDIT: Just found this in my 'building regs explained' book
The BCB may allow some flexibility in the calculation of the maximum area if there is a problem in achieving sufficient daylight in the extension. You might be lucky

You can do a SAP calculation if you need more than 25% openings

Thanks for the replies guys but to be honest I'm still unclear by your annswers.

Mikric, No under Nottingham.


Existing floor areas :-
kitchen 3.3 x 3.2m
living rm 1.9x3.2m
=16.64m2

Existing window (a single bay) :-
650x1300
1400x1300
650x1300
= 3.51m2


As per post #1

New floor area in the extension = 3 x 5.7 = 17.1m2

Proposed new windows = 12.62 m2

What is the are of new glazing proposed?

Some inspectors are rigid on the rule some are not.

If your inspector is rigid on the rule you need to either get SAP calcs done or reduce the overall area of the windows.

If you reduce the area down you just need to make sure that your elements (walls/floors/windows/roof) achieve the required U Values.

If SAP's are required they need to be done by a SAP engineer to prove the extension will comply given the large amount of glazing, maybe upping the insulation in the roof or/and walls in order that it complies. A decent SAP engineer will up advise as to what is the most economical upgrade.

Ok guys, sorry if I haven't made myself clear but at the moment I'm still in the design stage... so no brick laid or foundation dug.

FMT
Proposed glazed area = 9.1m NOT 12.62 which I quoted in Post #5


The first thing that I would like to understand is the 25% of the 'floor area'

What is the definition of the floor area :-

a) the floor area of the new extension only = 17.1m2

b) 'a' + existing floor areas (16.64m2) = 33.74m2

Please refer to my post #5 regarding existing floor area.

Unless I am wrong exceeding the 25% rule will kick in when your new windows area exceeds 7.8m².

Using nickjb's terminology I worked out as follows:

the maximum area of windows and doors and roof lights is calculated from:
Maximum area = 0.25 x 17.1 + 3.51 = 7.8

where 17.1 is the floor area of the extension and 3.51 is the area of any windows or doors that no longer exist or are no longer exposed as a result of the new extension.

Therefore you either need to:

a) Keep it below this figure.
b) See what the inspector says maybe he'll let it go.
c) Be prepared to undertake SAP calcs. (They really pretty easy to sort out BTW).

Thanks to everyone who posted. I now understand how to workout the 25% new-glazing rule.

After doule-checking my drawings, the extension area (2.7x5.7) =15.4m2

So with all your help, I calculate that my allowed glazing area = 7.36m2

With a proposed glazing area of 9.1m2 I will exceed by 1.74m2

Oh heck...

FMT would you kindly like to elaborate on, working out SAP calcs.

The following is basically an extract from a quote I did:

Standard Assessment Procedure (SAP) Calculations

Since 2006 it has been a requirement that all new dwellings have SAP home energy rating calculations prepared in order to demonstrate to local Building Control Bodies that the proposed dwellings meet the standards of the building regulations with regards to the Conservation of Fuel and Power (or heat loss). SAP calculations (as in this case) may also be required in order to support building regulations applications for existing dwellings - for example, when an extension is being added should the extension have a large glazed area. Basically in layman's terms because glass obviously has a poor insulation rating the SAP calculations will make sure that, overall the whole house is efficient in terms of heat loss and if necessary, other insulation being fitted as part of the extension will need to be increased to compensate.

The procedure is that you will need to employ a SAP Assessor to calculate the SAP Calculations, they will require a questionnaire be filled in with relevant information such as type of construction, type and make of boiler, size of and quantity of radiators etc etc. Once they receive your completed form they can carry out their calculations. They will then report back to you and between you both will 'beef up' the insulation as necessary. The SAP calculations are then submitted to Building Control.

You can get SAP calcs as cheap as chips online but you'll get little support if you need to beef things up or fiddle with the truth a little. Personally I always use the same bloke, he's not the cheapest but he makes it a very easy process to go through. My client paid about £200+VAT including an EPC.

However you're getting ahead of yourself, your inspector may not realise or care, even then he may not be bothered or may just ask for an increase in the wall insulation. Then again he could insist on SAP's.

FMT, thanks for that.

If the trifold doors you mention are the ones which B&Q sell then bear in mind that they have quite a thick frame thus the actual glazed area will be quite a bit less than the whole 2400x2100 as a good chunk of it will be frame. Or does one have to include frame in the 25%?

Yes

Build the walls with the doors and windows as required

When it comes to the roof, form the openings for the rooflights with double rafters and trimmers etc, but don't put the frames in, just tile and plaster over as normal.

Once the extension is passed off, put the rooflights in

If you are not of the disposition to do anything so bad as to contravene the b/regs, then do as per above but then just pay your £90 building notice fee and apply to put the two rooflights in - and then you get everything above board, certified and you don't have to conform to the 25% opening nonsense. And this will be cheaper than paying for the SAP calculations, or needing to upgrade insulation elsewhere

Thanks leew2, yes, I was looking at the B&Q dooors and I too was going to ask if I could 'minus' the frame as you mentioned.

But obviously FMT has answered already....damn! I thought I cracked it!

Woody, that sounds like a plan.

As I will be installing 3 rsj's, I am intending to make a 'full building application', so that I will able to order the rsj's confidently, (yes, I do have s.e. calcs but have come to opinion, not to take anything for granted when it comes to B.C.).

So can I just ask you to clarify that bit about the building notice, thanks.