It is always recommended that your Structural Engineer completes a physical survey of your home before producing the calculations and structural design for the extension. Some Engineers do not and instead make assumptions about the building that later need to be proved or disproved on site. Doing so is risky.
There are many reasons why you need a structural survey for your extension, and there are three reasons why an Engineer would make assumptions in lieu of physically surveying a property prior to crunching the numbers. First off, he may deem the proposed extension scheme to be relatively ‘straightforward’ and so not warrant a physical site assessment. A ‘straightforward scheme’ may be something like a single storey 3.0 meter deep kitchen extension, which has a couple of windows, a standard sized door to the garden and no internal walls coming down in the existing footprint of the house. A second reason for making assumptions would be if a strong precedent of similar development already exists in the immediate area, and so ‘what must go for one, must go for the other’, such as in a row of 20 Victorian terraced houses where 10 have already had their lofts extended. A third reason for making assumptions would be that the homeowner wishes to save money by not paying for a physical survey. There are many residential engineering practices that service this requirement, by taking the sausage-factory approach of churning out volume, quick turnaround work for a lower fee. They keep costs down by substituting a physical site assessment with working assumptions.
Any assumptions made will be marked on the drawings with the portentous words, ‘to be checked on site’, or similar. Left unchallenged, these assumptions are carried downstream and are surreptitiously inherited by the Builder.
Some of the classic assumptions pertaining to the structural fabric of a building include those made about the load-bearing capacity of internal walls; continuity of walls down through a building (notable for upstairs/downstairs flats); direction and span of floor and ceiling joists; foundation depth; soil type and the radial spread of tree root systems.
The design of whole schemes can be based on what is essentially educated guesswork. The risks of guessing wrong are high.
Let’s say that for a loft extension, an internal wall is deemed by the Engineer to have load-bearing capacity. The steel work is shown on plan to bridge from one side of the house to another, with size and span calculated just right, based upon having this internal wall to rest upon. The spreader plates supporting the beams are sized according to the thickness of the internal wall, timber floor joists are sized and spaced accordingly. All hunky dory on plan. Your Builders start work but soon discover that the wall in question is only 1-brick thick partition wall, incapable of taking the weight of the Engineers steel beams. Problem.
Work must now stop whilst investigatory work is carried out by the Engineer to figure out a work-around or devise a new structural design altogether. Doing so will need to be paid for by somebody and it will delay and disrupt the project timeline in a way you cannot control. For the Builder, pricing up the extra work from this unplanned departure from the project schedule will take time, time away from fee-earning work. This would potentially manifest in a costly bill for additional work. That’s if your lucky. If you’re unlucky, the Structural Engineer may conclude that in fact there is no viable work around to the problem. Or perhaps you struggle to now get hold of the Engineer to put it right because he is busy putting out fires elsewhere.
Scenarios like this play out all the time. This is why you need a structural survey for your extension prior to knocking up the calculations and structural design, as opposed to relying on assumptions.
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