Cost of side extension?

GDB2222

We are looking at buying a house, and we would need to put in a long narrow side extension, around 2.1m x 8m. Can anyone give a rough idea of cost please?

It's for a utility room/storage, etc, so fairly basic finishes. We are in London. It would probably have a flat roof, with a skylight. 

The house is 110 sqm, plus there's a double garage at the bottom of the garden. It was built 40 years ago, and there have been no alterations since. Would the extension come within permitted development?

It would need to come right up to the boundary. Is that a problem?  Next door is a cemetery, so at least there's no danger of undermining the next door property! 

Finally, how thick do the walls need to be to meet building regs? The 2.1m represents the external dimension, so the thickness of the wall is a significant reduction in the usable width.  

stuart45

The thickness of the wall depends on the method used to achieve 0.18 U value.

travis-powers

External walls are 350mm now with a 150mm insulated cavity.
 Can’t help with cost, labour and materials are still sky high, dread to think how much in London

FreeBear

Very rough ballpark figure - £3,000 per square metre. So something in the region of £50K plus the cost of fitting the space out. On top of that, you'll have architect fees, planning applications, and possibly Party Wall Agreement costs (depending on how close you are to neighbouring properties).

moneysaver1978

Walk around, and knock on the doors who have had an extension - they might be able to give you the names of the builders and if they are nice enough, they might even give you an idea of how much they paid.

GDB2222

Thanks everyone for your help. I was thinking £3k psm, so it sounds like I am in the right ballpark.

Would this be permitted development?

stuart45

You can use 100mm cavity with something like K106 cavity board if space is really tight.

travis-powers

stuart45 said:

You can use 100mm cavity with something like K106 cavity board if space is really tight.

That really interesting the job I’m on is having a similar extension, however I’m in Norfolk so costs will be a world apart, the architect gave my client the choice of 300mm build and 50mm insulated plasterboard or 350mm build, I’ll bring this up at the next site meeting!

stuart45

The price might put them off.

travis-powers

stuart45 said:

The price might put them off.

Our client is totally mad she had the trench blocks on site for a 300mm wall then decided she didn’t want insulated plasterboard and has now bought 350mm trench blocks, not my problem but our bricklayer is pulling his hair out!
 It probably would’ve been a cheaper option.

stuart45

Bad sign if she's changing her mind about the design before you've even got out of the ground.