Seven mistakes to avoid during the structural design of your extension

At the outset of design work on a house extension, homeowners are meaningfully involved in setting out the design brief and provide input to planning-level drawings. Then towards the end they find themselves the centre of attention as the working drawings are specified around their requirements. The stuff in the middle however, the structural work, isn’t the sexist part of the project. By this stage of reading you know what you need to know about the structural phase, but here are seven mistakes to avoid making during the structural design of your extension so to ensure this part of the project goes as smoothly as possible.

Mistake number 1: Not using the Architect’s recommended Structural Engineer.

Unlike finding an Architect or Builder, you probably won’t need to instigate a beauty parade of Structural Engineers for your extension because chances are, they will come recommended by the Architect. An Architect would only recommend an Engineer with whom they share an effective working relationship and with whom they have worked successfully in the past. If the Engineer performs poorly, the Architect suffers. So it’s in her interest to recommend the best Engineer for the job. By appointing an Engineer from the cold, you risk introducing uncertainty to the process; whether the two cooperate, whether they use compatible design software and whether they see eye to eye on the design.

As a side note, whether you opt for the Architect’s recommendation or otherwise, the Structural Engineer should be accredited members of an industry trade body; either the Institute of Structural Engineers or the Institution of Civil Engineers. This way to always have recourse to a governing body if needed.

Mistake number 2: Leaving structural assumptions unchecked.

Although covered at length previously, avoiding this mistake can save so much aggravation that it is worth a second mention. If the facts needed to number crunch are not available at the time of calculating, the Engineer may make assumptions about the building or proposed works. Where this is done, instructions ‘to check on site’ (or words to this effect) are usually slapped onto the drawings. Ideally nothing should ever be assumed but where it is, it must be fact-checked before work starts for someone downstream to pick up. If this level of fact-checking is not done until work starts and  structural scheme begins to unravel when it is, then you could face big delays and big costs as these issues are tackled by way of a structural redesign.

Mistake number 3: Presuming the work doesn’t require a Structural Engineer

By following the house extension process laid out you can be sure to have ticked all the necessary boxes prior to starting work. Once it does start, every homeowner’s temptation is to add more to the build schedule whilst the talents needed to get the job done are already on site. This is OK until you carry out an alteration that is structurally significant, but don’t run it through the Engineer first. Knocking out internal walls is a classic example. The Builder may well tell you he has the experience to order the right beam for the opening, and he may guess right, but nobody should make any structural alterations without consulting the Engineer first because the consequences of guessing wrong can get very serious.

Mistake number 4: Doing extra notifyable work without informing Building Control

Following straight on from mistake number 3, is to carry out further structural alterations and then not inform Building Control than you have done so. The usual suspects for this omission include widening external doorways and removing chimney stacks. Structural alterations like these require building control notification and inspection to ensure they have been carried out correctly and safely.

Mistake number 5: Keeping the Engineer in a passive role.

Very often Structural Engineers will produce the calculations and structural design, collect their cheque and that is that; their involvement in the project is over. For smaller extensions this may be OK, but there is value in keeping the Engineer more actively involved in dialogue in the run up to the build, because in the event of a major construction issue, the Engineer may need to play the starring role to get it sorted. Keep him primed and present in case he needs to leap into action at short notice.

Mistake number 6: Over-engineering for ease.

Where walls are being taken down and spaces opened up in the new extension, structural members (typically steel beams) are needed to support weight and transfer loads down to the footings. The precise sizing of these members is calculated by the Engineer. However, many Engineers are in a habit of over-sizing, just to be on the safe side. You may think it’s better to be safe than sorry, but on a job where every millimetre of space matters, unnecessary over-sizing of structural members could mean the difference between being cramped or being comfortable.

 Mistake number 7: Hiding structural drawings away in the calculations.

It is the Engineer’s job design a structural arrangement that works for the extension, so it is always a pity to see the fruits of this labour nestled away, hidden deep within reems of calculations. Here they sit disembodied from the schematic, serving a less useful purpose than if the structural arrangements were overlaid upon the approved plans where they can be easily read and interpreted across plan, elevation and sectional views.

So, avoid making these seven mistakes during the structural design of your extension and enjoy a smoother ride onto the next phase of the architectural process, the building regulation plans.

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