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What Is Under This House?

Updated: Apr 7


Have you been trying to buy the ideal family home but can't seem to find the right property? Maybe you’ve decided to buy a house, and then build an extension, to make it work for you.

That’s what my client had in mind when they contacted me to discuss this house in England. It turns out there are some hidden problems with the property that everyone should look out for when buying a home.


I have been a self-employed Architect in the UK since 2009. Alongside my regular practice, I also run the Real Life Architect website, where anyone can book a consultation with me to discuss altering and existing home or building a new house in the UK.

The driveway

In July 2023 I was contacted by a client who was thinking about buying this house. I have their permission to write about it but I'm not going to name them or give away the exact location. My client wanted to know what they could do with the property and they had three main goals.


  1. Convert the garage to a habitable space and change the internal layout to create a more open-plan living area.

  2. Build a small extension to the front to make that new living space larger.

  3. Take the roof off and build another storey on top with extra bedrooms.


The first two steps would be carried out as soon as they bought the house but the final step, adding a story on top, was not something they planned to do for several years. The house next door had added a story on top about 10 years ago and my client assumed this would be straightforward. Turns out it might not be so simple.


The house is part of this row, built in the 1950’s.

The first thing that struck me is, why are all the houses are set so far back from the street. The site is in a well-developed residential neighbourhood, there are properties on either side, mature trees and the large development behind the house dates from the same period but has a much higher density.

Why are these houses so far back from the street?

If this were an empty site, and a developer wanted to squeeze in as many homes as possible, it wouldn’t be difficult to build twice as many homes. So why didn’t they?


It turns out the client was from the area, they knew the history of the site and it raised some interesting issues. They told me that part of the site near the street had been used as a rubbish dump before these houses were built. They also told me that a stream used to run across the site but had been re-routed underground when the houses were built.

I should say that I have no way of checking the claim the site had been used as a rubbish dump but I did find the original planning approval from 1957 and this states the land could be used for residential purposes after “filling and leveling site” and “the watercourse running through the land being culverted to the satisfaction of the council”.

An excerpt from the original planning approval for the site

This would explain why the houses were pushed so far back from the street. There is an obvious issue with contamination when a building on ground used as a rubbish dump but, also, ground that has been made from rubble is unstable. If a stream was on the site, the ground will be a mixture of sand, pebbles, silt and clay. This means the ground might move over time, especially if a building is on top. It turns out, that might have been an issue with this property.


The client researched the local authority records and found that a building regulations application had been made to underpin foundations in 1991. I should say that I wasn’t able to see that document, it isn't available online and I am taking my clients' word on this. The trouble is, no completion certificate appears to have been issued for the underpinning work. This should have been applied for after the work had been carried out.

It might be that the owners never actually carried out the work to the foundations, or they never bothered to apply for the completion certificate but it could also be that the work was not done in accordance with the approved drawings. We just don't know, but it raises two problems for my client.


First, it shows that there might be a problem with the ground the house is built on. Why else would they need to underpin the foundations. Second, the Building Act states the owner of a property has sole legal responsibility for compliance with the building regulations. That means if my client buys the house, the lack of a completion certificate becomes their responsibility even though they had nothing to do with the work back in 1991. That's just how the system works.


I advised my client to have their lawyers contact the seller and have them make the application for completion, or provide some kind of indemnity to cover the possibility the work wasn’t done properly. This would require legal advice but, ultimately, if my client bought this house and tried to sell it again, future buyers might ask my client to solve the problem. It's best to just solve this now.


The internal alternations my client envisaged were relatively straightforward, even converting the garage seems feasible. Building immediately to the front of a house is usually not something planning departments encourage.

What the client envisaged building at the front

Many planning departments have guidance on this, and they typically don't allow anything bigger than a porch. In this case, however, given the house is so far from the public street, the odds of getting planning approval for a larger extension to the front are pretty good. It all depends on the size and appearance of the proposed extension, but I think it stands a reasonable chance. What will complicate this is that a common drain runs in the garden, immediately to the front of the house, right where my client wants to build that extension. It's the red dashed line on this drawing.

Drawing showing the below ground drains and water main

A common drain is a sewer that is shared by several properties. Even though it might be on your land, you can't just build on it. You need consent from the local water authority and they will need detailed information from your structural engineer. This is called building over consent, and it usually happens during the building regulations approval process.

That leaves the possibility of building another storey on top of the existing house, just like the next door neighbours did.

At this stage in the consultation, I couldn't ignore what we had learned about the ground conditions and the fact the house may have needed underpinning in the past. I told my client he should contact the neighbours and, if they were the owners when the house was enlarged, could they tell my client what structural challenges had to be faced to get this built.

The neighbours house is now much taller than the property for sale

You see, engineers usually design foundations to cope with the load of the building. They don't usually design for future enlargement and adding a storey on top will impose a lot more load on these 1950’s foundations. Even if the ground was solid, and we didn't have a history of issues with rubble and culverted streams, this would still be a problem. My guess is the existing foundations would not cope with the extra load and there are two options. Either the existing foundations need to be underpinned or a separate structure introduced to support the new first floor.

Underpinning usually involves making the existing foundations wider and deeper. Whenever Ive seen this done, the contractor digs a hole about one meter wide near the foundations, fits stainless steel rods into the existing concrete and then pours new concrete into the hole. The stainless steel rods help join the new and the old together. The hole is filled and a new hole is dug next door, and so on in sections, all the way around the building until all the existing foundations have been made wider and deeper.

The alternative is to leave the existing foundations alone and introduce a new steel frame inside the building to support the new first floor and roof. The steel column will sit on new concrete pad foundations, each around one meter cubed. There might need to be 6 or even 8 of these pad foundations inside the house, so lots of digging and disruption in the existing building. But it sometimes works out less expensive than underpinning.

In either case, this work will cost tens of thousands of pounds and, if all my client gets at the end of the day is two more bedrooms, it might not be worth it.


I advised them to negotiate with the seller and ask permission to carry out a ground condition survey. This involves drilling a core or auger down into the ground near the house, and examining what comes out. By looking at the type of material in the ground, and establishing its depth, the engineers can have a better idea of what kind of load-bearing capacity the ground has and whether the foundations need to be underpinned or reinforced in some way. I suggested to my client that they offer a sum of money to the seller so that the garden can be put back as it was if they back out of the sale after the survey.

The final thing to say is that anyone planning to carry out work more than a year or two in the future needs to accept that planning guidance, building regulations and construction costs will all change the longer they leave it before starting work. I can only advise clients based on what the rules are now and how much it costs to build today.

If you are planning to build a house extension anywhere in the UK, and you would like advice from an experienced architect, get in touch and book a consultation with me to discuss your ideas, timescale and budget. I provide a sense check on your ideas and can advise you on how to find the right local architect for your project.

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