High risk of subsidence flagged in searches - London FTB

ftbuk05

Hello,
I'm in the process of purchasing a flat in the SE London area and have had my searches back which flagged a high risk of subsidence based on insurance claims made in the same postcode area.

From what I gather the risk is mainly due to the type of soil which most of London is built on and last time I checked, most houses and flats in London seem to be holding up so I'm trying to understand how worried I need to be about this.

To provide context, I have not had a survey done as the flat I'm buying needs a complete refurb so I felt a homebuyers survey would just tell me what I already know. Having been to the property several times, there are no obvious internal cracks (the property is empty so I've checked thoroughly) however garages on the property (but not attached to the flats) were underpinned 20 years ago. I have asked what the reason for this was and I'm awaiting an answer but in the meantime should I get a surveyor to conduct a survey of the property to assess the risk of subsidence?

For reference, a few flats have sold in the block in recent months/years so clearly the subsidence thing isn't putting those buyers off?
If anyone has any advice, I'd be very grateful! Thank you.

Rankin21

I’d deffo get a survey done to put your mind at ease. 

In terms of the type of soil- majority of London, parts of Yorkshire as well as various parts of the UK are built on clay rich soils.  Which are inherently ‘weaker’ and can be massively affected by water levels. But like I said huge amounts of the UK are built on similar soils so on its own isn’t the problem. However there’s obviously something going on either In the more localised geology or something else which has caused other subsidence claims and the need for underpinning. 

So I think for me I would deffo feel more piece of mind getting a survey done to double check. It’s better you waste a few hundred quid on a survey to find there’s nothing wrong then waste god knows how much down the line when there’s an issue. 

Section62

ftbuk05 said:

From what I gather the risk is mainly due to the type of soil which most of London is built on and last time I checked, most houses and flats in London seem to be holding up so I'm trying to understand how worried I need to be about this.

What kind of flats? A conversion, or a purpose built block?  Year of construction? How many storeys? Local Authority or private?

ftbuk05 said:

however garages on the property (but not attached to the flats) were underpinned 20 years ago. I have asked what the reason for this was and I'm awaiting an answer

If they are just standard single-level garage blocks then it is likely they have very basic foundations compared to the flats. Foundation design is critical when dealing with clay soils, so it is feasible that the garages had shallow or slab foundations which were affected by movement, whereas the flats don't have the same level of risk (subject to the issues above).

I would raise an eyebrow that they chose to underpin the garages rather than demolish them and rebuild. The latter is often the more economical approach with basic buildings, so it would be worth finding out what factors were involved in the decision to underpin.

princeofpounds

ftbuk05 said:

To provide context, I have not had a survey done as the flat I'm buying needs a complete refurb so I felt a homebuyers survey would just tell me what I already know. 

A homebuyer's survey should tell you about signs of subsidence. I appreciate that this is a flat and therefore the scope of an inspection is limited to areas the surveyor can access, but it should have included a visual inspection of the overall building from the exterior at the very least. So I don't quite understand this sentence.

Subsidence risk is elevated across most of London, due to the soil as you say. I would always recommend a survey but I wouldn't be at all concerned IF it weren't for the fact that the garages have suffered already. As Section62 suggests the difference is probably due to the foundations however.

But it's unlikely you'll get definitive answers in the end. Generally-speaking, if the building appears ok and has been there a long time, it's probably ok.

Section62

Rankin21 said:

Which are inherently ‘weaker’ and can be massively affected by water levels.

It isn't a weakness as such, the problem is with certain types of clay having a property that they can shrink or swell depending on the moisture content. They are stable and have high bearing strength if kept at a uniform moisture level.

Rankin21 said:

However there’s obviously something going on either In the more localised geology or something else which has caused other subsidence claims and the need for underpinning.

Problems typically happen due to the building not having foundations which extend deeper than the level where the moisture content is unaffected by seasonal variation, or other factors such as trees or ground water abstraction.

If the moisture content reduces, the affected clays will shrink, and that will result in localised ground movement. Badly designed foundations cannot cope with that, which is when problems above ground become apparent.

ftbuk05

Section62 said:

ftbuk05 said:

From what I gather the risk is mainly due to the type of soil which most of London is built on and last time I checked, most houses and flats in London seem to be holding up so I'm trying to understand how worried I need to be about this.

What kind of flats? A conversion, or a purpose built block?  Year of construction? How many storeys? Local Authority or private?

I would raise an eyebrow that they chose to underpin the garages rather than demolish them and rebuild. The latter is often the more economical approach with basic buildings, so it would be worth finding out what factors were involved in the decision to underpin.

Thank you. The flats are 1970s purpose built, Four storeys high and private. And yes the garages are just single-level, so agree with you the foundations were probably just shallow hence the need for underpinning, but can I ask--would the decision to underpin as opposed to demolish and rebuild be a sign of anything else substantially worrying? Aside from poor money management perhaps on the part of the management company? 

ftbuk05

princeofpounds said:

ftbuk05 said:

To provide context, I have not had a survey done as the flat I'm buying needs a complete refurb so I felt a homebuyers survey would just tell me what I already know. 

A homebuyer's survey should tell you about signs of subsidence. I appreciate that this is a flat and therefore the scope of an inspection is limited to areas the surveyor can access, but it should have included a visual inspection of the overall building from the exterior at the very least. So I don't quite understand this sentence.

Thanks, I always was under the impression that the homebuyers survey would focus on the inside of the property, and honestly, the risk of subsidence didn't even cross my mind given that it's in London where there are thousands of properties next to it. But yes, the building looks sound from the outside (to my untrained eye) and when I brought my builder round there was nothing that made him shudder. 

princeofpounds

ftbuk05 said:

princeofpounds said:

ftbuk05 said:

To provide context, I have not had a survey done as the flat I'm buying needs a complete refurb so I felt a homebuyers survey would just tell me what I already know. 

A homebuyer's survey should tell you about signs of subsidence. I appreciate that this is a flat and therefore the scope of an inspection is limited to areas the surveyor can access, but it should have included a visual inspection of the overall building from the exterior at the very least. So I don't quite understand this sentence.

Thanks, I always was under the impression that the homebuyers survey would focus on the inside of the property, and honestly, the risk of subsidence didn't even cross my mind given that it's in London where there are thousands of properties next to it. But yes, the building looks sound from the outside (to my untrained eye) and when I brought my builder round there was nothing that made him shudder. 

The different levels of survey are described here:
 https://www.rics.org/globalassets/rics-website/media/upholding-professional-standards/sector-standards/building-surveying/surveys-of-residential-property-3rd-edition-reissue-rics.pdf

RICS likes to use level 1, 2, 3 etc. for it now, but basically level 2 is a homebuyer's survey and level 3 is a full building survey (which some people often still refer to as a 'structural survey' even though it's not performed by a structural engineer and is in some respects a misnomer as lower levels already consider the structure to some degree)

If you read between the lines, you'll realise that some of the bigger differences between levels 2 and 3 are not actually the inspection. The main difference in the level 3 inspection noted is that they will turn things like taps on and off! What you are actually paying for in level 3 is largely the written prose in the report containing advice about the building (Section 3.3 bullet points, basically).

Section62

ftbuk05 said:

Thank you. The flats are 1970s purpose built, Four storeys high and private.

Purpose built is good, typically more thought went into the basic structure than with buildings that become flats through conversion.

Four storeys is not high enough that piled foundations would be a certainty, and 1970's is before the time that movement of clay soils became a matter of serious concern and piling became the virtual default method of dealing with it.

Obviously you'd need a professional opinion, but from that information it is at least worth spending the money on the professional advice, rather than just walking (or running) away.

The main issue is going to be what liability (if any) you have if there were to be future structural issues, and also the difficulty in selling while any problems were resolved.

ftbuk05 said:

And yes the garages are just single-level, so agree with you the foundations were probably just shallow hence the need for underpinning, but can I ask--would the decision to underpin as opposed to demolish and rebuild be a sign of anything else substantially worrying? Aside from poor money management perhaps on the part of the management company?

No, nothing worrying. More an indication of what the original problem was and whether it was also relevant to the flats. If the garages are nicer than bog standard the rebuild cost could have been more than the remedial work. Or the specific design of the foundations might have meant that a ground stabilisation technique was feasible, and cheaper than traditional underpinning or rebuilding.

And if the garages were just badly built then the need for underpinning might be unrelated to clay soil movement.

ftbuk05

Thank you @Section62, very helpful. I'm not wanting to walk away at all, just want to ensure I do the right checks now to ensure I'm covered should there be any issues in the future as you say. I'm not worried about it falling down, just worried about insurance, liability and resale basically. 

AlexMac

Don't over-react.  You don't say which area of SE London, nor whose survey you commissioned, nor whether by "postcode (of past subsidence claims) " you mean huge tranches of our manor such as Blackheath SE3, Deptford SE4, Charlton SE7, Greenwich SE10, etc, or more local postcodes, nor over what period these claims arose, but...

.. we've lived in those areas for decades and are not aware of widespread recent subsidence problems.

OK we had a huge collapse of the A2/Blackheath Hill in 2002, but that was caused by hitherto unknown pre-Roman chalk mines so not a frequent risk!   And during the very dry summer of 1976, there was movement on some of the hills which run down from the stable sand and gravel substrate on the tops and plateux (the so called "Blackheath Beds") past silts, and clays to the floodplain.  If you drive down some hills like these i Greenwich, Westcombe of Charlton, you can see evidence of slight historic movement in the form of mild stepping in the facades and lintels of Victorian terraced houseswhere the underlying geology changes , but these buildings had booger all foundations.

(Geeks can search by postcode at the Geological survey maps at the link below to see how the geology changes along the contours where the river has cut down through these different sub-strate, presented as pretty coloured  layers along the contours!)
https://mapapps.bgs.ac.uk/geologyofbritain/home.html

So I doubt your 1970's block will have the same problem!