15 Dec 2011

How accurate are QSs?

I have a number of editions of your book and am now on the brink of my first self build. We have planning permission and detailed designs, and have had a couple of builders tender for the work, which was a condition from the bank in order that they would fund the project. However the costs are more expensive than I planned (surprise surprise I hear you say), and when I costed some of the major elements (ground works, external walls, roof) using your latest book, I came to half the price quoted by the builders. I have also got a few local sub contractors to quote and their numbers match mine not the builders
So I have employed a quantity surveyor to do it properly on my behalf, and am now the proud owner of 25 pages of costs for the project. Thankfully I am comfortable with this kind of numerical analysis, but I cam imagine at this stage most peoples eyes glaze over ! Unfortunately his numbers come in very similar to the builders (They would do wouldn't they I hear you say, they have both used QSs to cost the project and QSs will generally use the same process and hence come up with the same cost).
My analysis of some of the big numbers suggests that some of my QS's numbers are the same as the numbers from your book, but some are significantly more expensive (50% to 100%) more.
So my question is as a self builder what would your guidance be regarding the cost you can self build for Vs a cost given by a QS. Is there always fat in the QS figure to cover eventualities ? I guess I'm hoping for a shortcut rather than go through and cost the whole project using your book to come up with the relevant figure.
I want to self build but want to be ensure that there is a saving in there by doing this.




Good to hear from you. 

My feeling is that you are coming up against one of the age-old conundrums in building in that an awful lot of pricing is an art rather than a science. An expensive QS is probably about as accurate as you can get, but if you were to ask three other expensive QSs to undertake the same task you would get three different results.  And so it is with builders quotes. They will assess the size and difficulties of each task slightly differently and, crucially, they will assess the risk of these tasks being completed on time and on budget quite differently. The more off-beat the project, the more widely varied the quotations will be. My cost tables are really only there to provide the roughest of guidelines, not to construct quotations. 

One of the critical factors is risk - or more accurately the perception of risk. You have referred to it as fat which is how many people think of it. but its really a risk premium paid by you for them taking on the fixed price quotation. IE the risk of cost overrun is being transferred from you to them, and this is, if you like, insurance money for them taking on this risk. In theory, you can do away with this risk premium by becoming your own contractor/project manager, but you will only save money if everything (OK, most things) goes according to plan. If you prove to be a lousy or an unlucky manager, you may end up spending more money than the fattest of fat builder's quote.

I hope this helps. I realise this is very general advice, but as you note, it's a common problem which people undertaking selfbuilds face. We all want it to be costed out simply like a shopping list, but it almost invariably never works out like that.


Thanks for your response. I think you maybe underestimating the value of your cost tables ! In your book you actually break each of the tasks down into material and labour which makes it easy to add up the totals. However the QS uses a fixed rate for tasks which includes labour, materials and equipment (i.e. £24 per sqm of blockwork - which I think is spot on for blockwork but some of the others seem very high). This makes it very hard to break down to see what is actually costed without working out the materials. Indeed the QS bemoaned the fact that in the old days he use to produce a bill of materials but now no one wants it. They just use the fixed rate to work out the costs, and then only ever plan one stage ahead. However he is very reluctant to produce a bill of materials for me.
I've just re-read your chapter on project management and it all rings true. I would just love to have the complete bill of materials so I could see once and for all how much the materials would cost Vs labour. If I use the QS's 40% is material and 60% is labour, then if I assume each subbie earns £200 per day (quite generous ?) then the labour cost is the equivalent of 10 people for a whole year !




Thanks for the interesting feedback. I spend a ridiculous amount of time on these cost tables and rarely get any feedback on how people use them.



  1. When we did our self build we didn't use a QS, partly because when I went to open an account at the builders' merchant and explained what I was doing, they said 'send us a copy of the plans and we'll price the material for you'. I sent them the plans, not expecting much but quickly got back quantities of materials and costings that turned out to be very useful and surprisingly accurate, and all for free!

    I agree with Mark though that pricing always involves a host of assumptions, unknowns and risks. You can't know what it is going to cost before it's finished, however much you want to, but you can, for a price, reduce the uncertainty. Probably 'top of the range' is getting an architect to arrange and supervise everything - you'll get a nice house to a fixed(ish) price but one that'll make your eyes water. A single main contractor will reduce the price and the level of certainty, etc.

    We decided the promise of a lower overall price made the uncertainty worthwhile so we bought our own materials and hired our own subbies. It worked well for us. Good luck whatever you choose.

  2. Mark is correct - but there are a couple of other factors that influence contractors' prices too.

    For small contracts such as this, many small builders won't accurately calculate all the quantities - it's an expensive process with no certainty that they'll get the work. For many elements they'll just take a look at the building size and complexity and 'guess' the number of days it will take them based on past experience plus the rough materials cost - plus an error margin.

    Provided they think you're serious about getting a contractor to build your house (rather than using them as a price comparison service), larger contractors will tend to calculate the quantities (though they would prefer a Bill of Quantities), so their internal pricing will generally be more accurate, but they'll also have higher office overheads to recover in their quotation.

    And, depending on the amount of work they have, all contractors will also vary the margin that they add; if they are busy (or expect to be) they'll quote higher prices irrespective of most other factors, if not it will be lower - though their perception of you as a client may also influence this.

    As a result of such factors no price book or quantitiy surveyor can calculate the 'real' cost.

    Had you asked the contractors to price a Bill of Quantities, one potential advantage might have been that you could have used it to haggle with them over elements that were siginificantly higer than the QS's projections - but they may not have been willing to do so.

  3. Having tried a couplev of times in my long architectural career, on smallish highly detailed schemes, I have yet to receive a BQ that was worth thr paper it was printed on - staggeringly sloppy mis-measuring, part several times over, part missing out whole chunks incl usually the best designed-for -economy bits. I tried to believe this was black-art wisdom that wd make sense and get an end-of-day realistic price from tenderers relieved of having to take their own quants, but no such luck, had to (twice) abandon the QS route and revert to tried and trusted full spec/drwgs priced by ea tenderer. Grrrr.

  4. A good QS will let you have their underlying data, the details that they used to build-up their bill of quantities or cost estimates for you.

    In arriving at their estimate of costs, the QS should have used published price books, such as Spons or BCIS/Wessex. The next stage of a QS's work would be to produce a blank bill of quantities to issue to several contractors to enable them to price the project and then the estimate provided to you could be compared against the cost estimate from the contractors.

    The estimate and the returned quotes can only really be compared if there is an original breakdown of the components of a quote, something which the major price books provide.

    Getting several different QS's to price a job may give several different results, but only within a small margin of error on the measured works areas because the measures of materials are factual (if there are good drawings) and the published price books are common to everyone, so I hate to suggest it, but Mark might be a little off the mark on that point.

    Basically, for a QS a cost plan or estimate for a house it not rocket science and they should be open with their build-ups if you ask them. If there are going to be major differences between several QSs estimates or in the data behind their work then it will be on lump sum areas where they could not get published price data, such as lump sums for specialists kitchen equipment, specialists packages, etc.

    QS's won't allow 'fat' in their work on measured items, but they will allow some element of uncertainty in prices but this will be shown on its own so you know what it is.

    If your QS is a member of the RICS, then they should have given you some form of agreement for services to sign, if you take a look at that you might see if the work they undertake for you actually belongs to you - unlike architects who tend to own the intellectual property on any designs they undertake on your behalf.

    If you just want a rough idea of costs at an early stage or to compare against, you could use Sirca Home Developer which is a software package that lets you play around with some of the building costs from its own database and links the costs back to the overall costs of a development, including VAT and finance costs. Use this with Mark's book and you'll get a good idea of budgets and what people are saying to you.

    After several failed attempts at trying to start a project (can get engaged or even to the Church, but can't seem to get married), you get to pick-up a few things along the way! Still, land seems to be getting cheaper at the moment, so there is still hope and the failed attempts might have been for the best.