Erm .....b) I think.

Im not telling you where I work, but with a small extension, I would ask for better Uvalue floor or wall or roof say, to allow you to have your big windows. And / or I would perhaps ask your to up grade you loft insulation to current standards to off set your big windows.

Hi Justin.

I agree with the a) option. I have obtained many approvals on this basis, but don't forget that the U-values for the "covered over" openings added to the 25% of floor area are fixed in the regs. Having better than minimum U-values for the new extension can sometimes help.
I have pruduced a spreadsheet for calculating and printing out to send to building control that helps me play with the figures. You are welcome to have a copy of if you can send me an E-mail address. Mine is : charlieduke@tiscali.co.uk

The full SAP 2005 calc can be used if you are carrying out improvements to the main house at the same time, ie adding insulation or updating the boiler. I recently gained approval for a client's glazed kitchen extension because the old 65% eff boiler was being changed for a 92% one, saving massive carbon emmissions even with the high loss extension.

Good luck.

Thanks for the replies. I see from your comments (Thanks Joinerbird and Helen) that it's perhaps not always handled the same way by different.. er.. inspectors. I think I'm encouraged overall that I can use (a).. which might just see me through if I spend enough on the glass!

Just to answer some other points.
Joinerbird: Loft insulation might normally be a useful offset if I were to go for the whole house SAP, only I have already (4 years ago) increased my loft insulation to about 400mm, so it might be pretty hard to claim this as a credible offset against my planned extension! I have also put in a woodburner but I did this also 18 months ago, and again it might be hard to use this for any whole house offset SAP calc as an offset against the starting point. Having done this, the next economic method if I were to use the whole house SAP offset would be to improve internal wall and ceiling insulation round my integral garage (single brick!), and after that by replacing existing DG windows with better ones. I'd like to and keep the extension as a self-contained unit for method (2). (I have actually got quite a long way with SAP form already, but would like to keep this only as a backup if I can't make the numbers work by the second method).

Charlie: yes, my U vales for the "real" extension are (where I can do it) going to be better than the part L1b requirements, whilst my "theoretical" extension uses the tabulated figures (Including the dreadful yet useful U=3 for a front door!!). This all helps the cause. Regarding the spreadsheet, thanks, I have been doing my own spreadsheet for these calcs, but if you have a proven format which has been accepted, then I might find something from that useful. I'll drop you an email. Rgds.

Chris; thanks, yes. As I mentioned explained. I would have had ample scope for offsetting the extension using (almost ineffective) previous loft insulation with 400mm I now have. Yes it costs very little, Only doing this 4 years ago may not hold water as an effective offset for a futeur extension.

So Helen and Joinerbird: Would I have any credibility trying to offset existing old work against my extension in SAP? I am not seriously thinking of trying this one on, (though I might if given a hint it could work). I'd sooner contain the job on it's own, and hence my reliance on tight calculations for the extension.

Justin

Why do want so much glass in it any way aren't you going to be trying to heat the gardens?

Hi PaulT.

Thanks, Yes these are similar numbers to ones I'm working with. I'll continue to work on keeping SAP out of it then if I possibly can. I agree that it seems a little contrived to use the pretty poor minimum standards in L1b as a tool to make the real extension look good. - From a practical point of view i'll continue to improve the rest of the place wherever I can. I also hope to gain some winter solar gain from the glass.

For Tony: "...aren't you going to be trying to heat the gardens? ". Good point. I don't want to do that, and am striving for the right compromise. Have a think about the picture (attachment), and picture this though.. This is the only side of my house with a proper garden, it faces South, it's elevated and it has unobstructed views over 80 Ha of rolling deer parkland, it has no road on this side, the view is about 20 Miles, it's my garden, there are no windows.

Blame 1970's builders, but we bought the place almost entirely for the location and would like to try and do something. Hence some glass!

Suggestions welcome, but it's a major overhaul to upgrade the flat roof kitchen, removal of porch and 1970's internal door+glass. Replacement with and about 24M^2 added floor area/kitchen in SE corner, with up to 15M^2 of combined glass+door area. Juggling with the aesthetics versus the glass area versus my concience. It looks just about do-able with L1b, but I need to use offsets allowed for as we've been discussing, good insulation and some good glass. Winter solar gain should be helpful, but that wouldn't help for anything as far as regulations go unless I go the whole hog with SAP.

Rgds

If the views are so good, how about a flat roofed extension with roof garden accessed through a new a door from the upstairs? I'd extend the flat roof a couple of feet to create an overhang above the south facing windows else you'll roast in summer.