Cost of extension - Builder or Project manager

12 Jun 2007
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United Kingdom
Hi Guys,

I have read various estimates for extensions saying that they tend to cost around £1200 per sq meter.

We are looking have a 3m x 8m extension and trying to decide on whether to employ a builder to carry out the entire project, or to project manage ourselves.

Whilst not experts we have carried out a few renovations, and have contacts for most trades who have proved reliable (other than a groundworker for site clearance, drainage, foundations etc although I am sure we could find one).

I assume a builder is in effect a project manager and although may do most of the work, will probably bring in other trades as and when needed, but will add on, I assume, a reasonable profit.

If the average quote from several builders is for example £30,000 for my extension, could you give me an idea of the 'profit' element of this, ie if I project manage myself, what sort of saving could I make.

My architect has offered to assist me in the project management by visiting the site once or twice a week as required throughout the build at £100 per visit to check on key stages etc.

I would be grateful for any thoughts, Cheers!
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The cost of full project management is normally 10-15% of the project cost. In my case this would be £3-4k for full management.

On the basis a visit of an hour will take him 3 hours with travelling, and with the build taking about 6 weeks, this option will coat me about £1200 to ensure the plans are followed correctly.

Is £100 unreasonable for 3 hours?? My dentist charges £100 for 30 minutes!
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On a small extension like this, your cost per sq/m may well not be as much as you have quoted. You can't fit to many extra's in that size of space.

A decent contractor should wrap all of that up on his own, no real project manager needed. A small domestic contractor will have many contacts he uses regularly for specialisms, and cover most of the other work his self.

Building Control will check at regular intervals as well. One contractor - one point of contact - make sure you have a fixed price contract with them - make sure your extra's lists are priced in advance before they start them and authorised in advance - make sure you have a retainer built in the contract.
+1 on this.
Presumably it's a single-storey extension, with lean-to roof?
If so, it's a relatively straightforward build. As above, the local inspector will check the main points, such as the ground before the foundation is poured, and they usually err on the conservative side.
They will also look at drainage, and usually do a 'pre-plaster' check to ensure that the rafters and insulation are OK.
Your architect will probably recommend a SE if you are opening up and putting steel beams in, and the council will check the figures for this.
Other than that, there are only likely to be minor 'snagging' points which reputable builders attend to anyway.
Rather than an architect hovering in the background, your best bet is to get a good builder and that is usually by recommendation.
F**k me, I wondered why you can never get an architect on site, you have to give 'em £100 quid first!

I wonder if he will turn up with a flunkey to hold his brolly and help him into his pristine boots and high vis?


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