Double storey rear extension

cyclonebri1

Furts wrote: »

I was on a site this morning - it got regulations in late 2010. 50mm Celotex with 50mm clear cavity. A site at lunch time was on a more recent approval - 60mm Celotex with 50mm cavity. A site this afternoon has just got approval on 75mm Celotex and 50mm cavity.

This variation represents less than three years. This why I suggested to OP building on the existing extension would, on a balance of probabilities, not be possible. Yes, there are always solutions, but for an extension this is unlikely to be viable.

I accept my view was condemned by a number of fellow mse colleagues - but we are all entitled to our opinions!

It's wrong to condemn any alerts to future possible problems, but some posters seem to take joy in it, if my view came across like that then I apologise. My main point was the foundation issue, that was from personal experience.

The wall bit, I was suggesting there may be an alternative, I didn't realise you have to have an air gap when using cellotex??, the last involvement I had the regs demanded 100mm rockwell, a complete cavity fill.

Do you not think that if the foundations prove OK, the local authority will "cut a little slack" in this situation?.

Furts

Do you not think that if the foundations prove OK, the local authority will "cut a little slack" in this situation?.[/QUOTE]

Yes - I have discussed and traded off items with Building Control. It has to be a reasoned, logical approach.

No - Building Regulations are the minimum standard required by law. Extensions are required to meet this minimal standard whilst complete buildings (including new houses) are required to meet higher standards. As such, it is impossible for personnel to lower requirements for extensions without breaking the law.

stebiz

Thanks for your views. Much appreciated.

MX5huggy

There are at least 2 easy ways round any wall uvalue issue.

Use Xtratherm http://www.xtratherm.com/applications/cavity-walls-full-fill
Basically Celotex you don't need a cavity with.

You can stick up any depth of insulation on the inside under plaster board.

Or the inner skin of blocks could be Celcon Solar or similar insulating blocks.

You can trade insulation elsewhere in the extension showing that the area weighted uvalue is better than if you followed the minimum standards.

Finally you can get a SAP Calculation done (around £250) to prove that the newly extended house will have lower carbon emissions than if you did insulate as required. May be by fitting better loft insulation in the existing house, or a wood stove, new boiler, new controls, new windows etc.

Furts

MX5huggy wrote: »

There are at least 2 easy ways round any wall uvalue issue.

Use Xtratherm http://www.xtratherm.com/applications/cavity-walls-full-fill
Basically Celotex you don't need a cavity with.

You can stick up any depth of insulation on the inside under plaster board.

Or the inner skin of blocks could be Celcon Solar or similar insulating blocks.

You can trade insulation elsewhere in the extension showing that the area weighted uvalue is better than if you followed the minimum standards.

Finally you can get a SAP Calculation done (around £250) to prove that the newly extended house will have lower carbon emissions than if you did insulate as required. May be by fitting better loft insulation in the existing house, or a wood stove, new boiler, new controls, new windows etc.

This is sound advice.

But if these upgrades are undertaken it may be cheaper to knock down the existing extension and rebuild with exactly what is now desired.

cyclonebri1

Furts wrote: »

This is sound advice.

But if these upgrades are undertaken it may be cheaper to knock down the existing extension and rebuild with exactly what is now desired.

I think what I/we/others were getting at Furts is that the cavity size, hence overhang issues is irrelevant, it can be overcome. The building reg changes are about insular values not constructional stability.

There is no way I can see that it's going to be cheaper to knock down the existing and rebuild, the footings would likely need extending in a width for a start, given an extra 50mm cavity requirement don't you think?

Furts

cyclonebri1 wrote: »

I think what I/we/others were getting at Furts is that the cavity size, hence overhang issues is irrelevant, it can be overcome. The building reg changes are about insular values not constructional stability.

There is no way I can see that it's going to be cheaper to knock down the existing and rebuild, the footings would likely need extending in a width for a start, given an extra 50mm cavity requirement don't you think?

If the extension is poorly built, or poor in terms of energy efficiency, or if substantially glazed, or if lintels above openings will not take additional loadings etc. the answer could be to start again at slab level.

An extension can be built for remarkably little if you are careful. I have just had four built on my home. Three single storey extensions, a two storey extension, internal walls, joists, boundary wall, chimney, fiddly labour intensive window cills, brick banding etc. The bricklayers labour for a very good standard of work, came in at about £8000. I wanted local UK tradesmen - not quotes from cheap labour from oversees. I was prepared to pay the price this entailed. Even so, I believe this represents value.

I supplied the 8800 bricks and even I (cynical and world weary!) still cannot believe any company could fire up a brick kiln for the price the bricks were supplied to me for. Similarly, even the men in a nearby steel fabricator could not entertain the price the lintels were supplied for. Upvc windows, plasterers, carpenters, roofers, artexer all amazing value and good. And everything was UK sourced.

Returning to the thread...the foundation detail could probably be resolved by altering the line of the facework - move it out slightly, or by introducing a cavity tray above slab level. Even a corbel detail on the inside leaf may be possible, or build the inner leaf in a wider block. There will be a solution.

keystone

Why would a builder dig the founds deeper than necessary for a single storey extension in the first place? The answer is he wouldn't. We can argue to the cows come home about whether the founds can be extended sideways to accommodate additional width and whether the upper storey can be made to line up with the lower one due to BR changes but if the original founds just ain't deep enough in the first place for a two storey extension its all doing the usual up the wall in an effort to find the highest attainable point.

Cheers

aggypanthus

This is interesting to me as I want a dble height ext at back of the house, as opposed to the side, how does the roofline match the old roof? when adding on at the back?

System

keystone wrote: »

Why would a builder dig the founds deeper than necessary for a single storey extension in the first place? The answer is he wouldn't. We can argue to the cows come home about whether the founds can be extended sideways to accommodate additional width and whether the upper storey can be made to line up with the lower one due to BR changes but if the original founds just ain't deep enough in the first place for a two storey extension its all doing the usual up the wall in an effort to find the highest attainable point.

Cheers

depends on what the ground conditions are surely? the depth/type of foundation might be fine, the cavity walls might be fine to achieve uvalues etc - with the limited information the op has provided no-one can say.

Op, yes it can be done if the conditions are suitable, you need to have someone come to inspect to see what is possible in your situation