Home Extensions – Top Ten Mistakes Made by Builders

It might look nice but did your builder avoid the most common mistakes whilst creating your home extension?

Mistake 1 – Excavations & Foundations

The first part of the home extension is usually to with the foundations and you will be surprised to learn that at least 25% of the foundations are not positioned directly under the walls on top. Excavating off centre or in the wrong position is quite common and in one particular case I had a builder that excavated for a two room side extension on the wrong side of the line that was set up. His ignorance doubled the cost of the excavations and foundations at the drop of a hat.

Mistake 2 – Damp Proofing Course & DPC Membrane

This part of you home extension is a minor job but of vital importance. If it’s done wrong the damp enters the new walls within a matter of weeks and over a year or causes severe damp problems.

On one site I inspected a few years back the builders had used plastic rubble bags to form a damp proofing course instead of using the correct width dpc roll.

Mistake 3 – Extensions Are Rarely Square

The vast majority of home extensions do not have 1 square corner. At least 98% of builders that I have met in the last 30 years didn’t have a clue as to how to check if the home extension there were building was in fact square to the property.

Mistake 4 – Services in the Floor

Time and time again I have come across copper pipes and electrical cables that are run directly in the concrete floors without any form of sleeving or protection. Copper pipes corrode quickly when encased directly in concrete as the cement practically eats the copper. As for the idea of electrical cables running around live within the concrete without any sleeving well that just make the hair on my neck stand up.

Mistake 5 – Windows & Doors

You might think this is an easy job but once again it is pretty common that the height of windows and doors are often comprised because the height of the floor was laid wrong or the bricklayer made a mistake. Ideally all you should see in a doorway to the garden is the wooden threshold and not a little wall under it. In some cases you don’t even see the threshold because the frame was set in too low and this equally is simply poor workmanship.

Mistake 6 – Floor Heights

New floor heights are amazing in the various ways they can be botched. Probably the worst scenario is when they are too high usually resulting in a step up from the main house all because the builder didn’t drop the ground a bit more when building the extension.

Mistake 7 – Ventilation To The Sub Floor

When you build a side or rear extension to an older property with ventilated sub floors it is essential that you continue to provide ventilation through the new extension floor. It’s a simple task that costs peanuts but eight times out of ten the builder doesn’t bother and the end is result is usually dry rot within a few years.

Mistake 8 – Insulation 8

A few years ago extensions were built with little or no insulation. I’m pleased to say times have changed and that you are now required to have insulation within walls, ceilings and floors. Most local councils now insist on additional dry lining with an insulation backed board to breeze block walls so make sure you builder conforms. Also make sure that they put at least 100mm of insulation into any concrete floors. If you don’t tell them they wont bother and building control may not pick it up.

Mistake 9 – Drainage

This has to be one of the easier tasks that invariably gets done badly or in fact missed out as much as possible.

When you add a full width extension you must always allow for the rainwater from the main roof. Time and time again you see that this water is left to discharge onto the extension roof which it is simply not supposed to happen as it will always lead to premature problems.

The other point of course is that rainwater is not meant to discharge into soil stacks and has to have it own means of going in to the sewer system or soak-away. If your builder gets this wrong its usually a huge problem to put right at a later date.

Mistake 10 – Encroachment

I left encroachment until last mainly because its an unfamiliar word to the builders and to most homeowners but can be a huge nightmare if it occurs on your build project.

Encroachment is where you build over the boundary line. Most people think this relates to the physical ground only but in actual fact time and time again you see pipes, gutters and cappings that clearly encroach on other properties which can be a nightmare to put right.

In most cases the builder carries out the encroachment in pure ignorance but it can leave you with a hefty bill to put right or in fact legal fees should you get sued by your neighbors.


The best way to stay ahead of the game is to be informed.