Hi,
We're planning on sticking a small extension on the back of our kitchen. From checking the planning portal site it looks like I only need Building Regs, and not Planning Permission.
I'm currently intending to draw up the plans myself (this may change if/when I get out of my depth), and I have a friendly builder who's helped me with a few details so far. Despite his help I've hit a few questions that I was hoping someone here may be able to help with, or point me in the right direction for...
[b][i]Question 1 - Stepping in[/b][/i]
I'd planned for the extension to be the full width of the kitchen, with the bricks keyed into the existing brickwork at the corners, lengthening the side walls. The builder mentioned that extensions are often slightly narrower (half a brick) than the building they're joined to.
Does anyone know if this is a mandatory requirement, or if there's a reason why this is done?
Essentially are building regs likely to pull me up on this if I don't step it in?
[b][i]Question 2 - Cavity thickness[/b][/i]
New builds all seem to use 100mm cavities, however as this extension will be on the back of an older house I'd like to stick with the existing cavity thickness (60mm) to avoid the internal wall stepping in. I'm hoping there is a 60mm cavity wall insulation that's rated to the same level as 100mm that I'll be able to use.
Anyone know if Building Regs people are likely to allow this?
Essentially the kitchen isn't too wide, so anything that causes it to step in a few inches (both items above) may make it not worth doing...
I'm bound to have further queries as I go along, but these feel like the main hurdles at the moment.
Cheers,
V
Q1 No, it makes building easier, but there's no requirement.
Q2 100mm seems huge! Most of my extensions are fitted with 30mm insulation (look at celotex boards). I strongly suspect you'll be OK but ask the question before you build - only your specific local authority and inspector will be able to answer what he'll let you get away with!
no offence but sounds like you might already be out of your depth! i'd get someone to do the plans for you.
1. stepping the wall in depends what you want to achieve visually some do some don't the half brick is random statement. read the permitted development guidelines for your LA and decide what you want to see/achieve.
2. insulation thickness is dependent on required u-values, simply matching the existing cavity won't work its a calculation and will probably n require additional on the inside of the wall therefore defeating the object?
Question 1 - Stepping in
I'd planned for the extension to be the full width of the kitchen, with the bricks keyed into the existing brickwork at the corners, lengthening the side walls. The builder mentioned that extensions are often slightly narrower (half a brick) than the building they're joined to.
No building regs/planning requirement for this - the extension we built 4 years ago is parallel to the existing building.
We'd originally planned to key the extension into the existing bricks, but on closer inspection the existing brickwork spacing was very shoddy, and our builder wasn't keen, so we just butted up and hid the joint with a strategically placed downpipe.
Question 2 - Cavity thickness
Our extension walls are thicker that the existing building, however this only noticed on one wall so our builder plasterboarded/skimmed the existing wall to bring it out to the same thickness as the extension - this was important to us as we had a run of kitchen cupboards/worktop along the whole wall.
1. Conservation areas sometimes req this, ours did, we had to make it visually different to the original house. It was 1820's though !
Thanks for the comments, it's all helpful.
None taken, I know I'm skirting close to the edge of my comfort zone/ability. It's an area I almost pursued as a career and have an interest in, so if nothing else it'll be something geeky to do before admitting defeat and using a professional 🙂no offence but sounds like you might already be out of your depth!
Tie the new brickwork into the old using something like a [url= https://www.strongtie.co.uk/products/detail/crocodile-wall-extension-profile/313 ]Crocodile[/url] - make all the various junctions much easier to achieve.
If you could get 60mm insulation that worked as well a 100 then builders would be using it as you'd get more floor area for the same footprint.
We typically spec 150mm cavities with 100mm insulation or where builders are happy to use a fully filled cavity then 100mm in total.
If you could get 60mm insulation [b]that was cheaper than[/b] [s]worked as well[/s] 100 then builders would be using it
We are using 60mm floor insulation in a commercial premises as defined by a BR Officer, due to wanting to keep it as thin as possible, but its no cheaper than 100mm.
You need to meet the requirement of the Building Regulations. Part L1B specifically for conservation of fuel & power. To save you looking, you need to hit U value of 0.28 or lower for your wall construction.
Kingspan website has an insulation calculator which will give you a pretty decent idea of what thickness you'll need for your wall construction. Assuming brick outer, medium density internal blockwork and partial cavity fill you'll likely need at least 40mm of insulation to hit a U value of 0.28 (domestic extension target).
Handy links: [url= https://www.uvalue-calculator.co.uk/ ]Kingspan Calculator[/url]
[url= https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/540327/BR__PDF__AD_L1B_2013_with_2016_amendments.pdf ]Approved Doc L1B[/url]
Jeff
Thanks ffej, that's really useful.
I'll have a proper look at L1B later, but the calculator suggests I'm down to 0.25 with 50mm insulation.
Q1 - It is usually better to have a slight step/ check or visual break between external walls of different materials for aesthetic reasons. Never try to match old with new, it is very hard if not impossible to do, you are better off making a feature of the 'join'.
Q2 - Consider an all timber wall which will be thinner overall, or possibly consider a replan. If you are doing it at all you might as well maximise the benefit.
Most rigid insulation boards need 50mm clear cavity so factor this into your wall thickness calls.