Can you make a semi-detached BISF property Detached?

Can you make a semi-detached BISF property Detached? 1

Hi, I live in a BISF property and for the past 4 years, have been dealing with my insurers with regards to subsidence. After years of investigations and subsidence monitoring, it appears that the main culprit is the neighboring property which has substantial subsidence and is pulling my property with it.

BISF house cad style drawing
Library Image

As yet, it’s not clear how the neighbor will remedy their property (and mine for that matter) however one of the options made available to them is to demolish.

My question therefore is whether you can demolish a semi-detached property and not affect the other ?(my property)

I’ve not come across any situation where this is the case as my understanding is 2 semi-detached BISF properties share the same structural frame and concrete foundation.

Is it possible to demolish one and not the other?



  1. Hi all. As it’s been a while, here is an update for you.

    The loss adjustor has conducted his inspection alongside their preferred construction contractor and has, as of last week, issued a ‘Schedule of Works’. They have asked me to agree to this before work can commence. Although the SoW seems to cover all the visible snags around the property (as pointed out my me), predominantly plaster cracks, they seem to be overlooking the likely snags that are not so visible, namely the slab foundation and steel structure.

    I have highlighted this to the Loss Adjustor and his view is that the property is now ‘settled’ and is unlikely an further movement will occur. From your genorous comments made by you all in the earlier posts especially how Insurers are keen on a low-cost remedy, I have challenged this and am awaiting further info. I have requested that, somewhow, the whole foundation and steet structure is examined for any imbalances on the loads from the steel structure which inevitably would impact the foundation strength.

  2. Wow, that is a noticeable tilt on the house on the right! I’d be surprised if it was just the trees causing that, though no doubt they didn’t help. I would expect the problem is down to something more fundamental, such as instability in the ground that wasn’t corrected when the foundations were laid.


  3. Hi Roger
    Take a look at this video, it’s on 4 OD. It’s a case where Sarah Beeny is helping the owners of a house that has been hit by tree roots and in this instance the house actually did re-settle.
    Although the house in the video is not of steel construction, it does give a valuable insight into the effects of damage, movement and it even deals with tree preservation orders.

    BTW, should anyone consider re rending any part of the exterior of your property, they should not use just any old standard render as the the coating needs to have a certain amount of flex. This is partly due to the fact that the steel frame can expand and contract considerably depending on the weather.

    I should also point out that not every foundation of a BISF house was built in the same way. Some were complete slabs but others were not.
    A good example of open grid style can be seen below.

  4. Thanks for all your comments. I hope to get some pics to you shortly.

    Both properties are situated on a slight gradient. Not much of a concern as nearly all of the other BISF properties on this street is on a gradient. My property is though at the top of the hill at the end (or start) of the street. There is no evidence of any mining ever being conducted in this area so concerns on that front

    The main cause of the subsidence was due to a number of conifer tress in very close proximity to the neighboring property (Located alongside the front to the back of the property on adjacent land next to the offending property,a pub car park). Various soil samples were taken outside my property which proved which trees were causing the subsidence. All the conifer trees have been removed, stumps included, however there is a battle to remove another offending tree, an Oak tree, located further into the car park however there seems to be a preservation order on it and as yet, this tree has not been removed but the Pub owners have been instructed to remove.

    The Loss adjuster has carried out an assessment of my property and have drafted an ‘schedule of works’ to remedy the various, predominantly plaster cracks, around my property however I sense they have no experience with BISF Properties as they’ve not taken into account the structure. When quizzed about this, their view was that once the property had ‘settled’ or ‘stabilised’ the structure should return to it’s default location. Really ?

    The offending property is currently inhabitable due to the subsidence. The property is council -owned, On one occasion I managed to gain access to the neighboring property due to a workman attending the site to make the property safe from a gas leak (Metal thieves) On entering the lounge I noticed a huge tree root making it’s way through the concrete foundation and effectively pushing each side of the foundation upwards by a gradient of 10-20 degrees. The foundation crack runs from the lounge window all the way to the opposite wall. Not sure if the crack continues into the dining room but have to assume it does as there is a wall crack at the rear of the property

    My Insurers/Loss adjuster’s view is that once my property has settled, they will fix all issues with my property however having advising them that the property next door may be demolished and that potentially the owners may decide to buy me out, the insurers are still willing to remedy my property even if it’s earmarked for demolition. I seem to get a lack of response when I ask whether they have contacted the insurance company representing the council and what they’re plans are. It seems that because the property is council owned it has to go through a process of cabinet meetings and hence why it’s taking them so long to reach a decision.


  5. What a Pickle!

    This post is a little ‘Long-Winded,’ however I hope that the first section is just as useful as the latter. Below are two options to consider, for remodelling a BISF Semi, into a Detached. Whilst both options treat the ‘Effects of Subsidence,’ it is worth exploring the actual ‘Cause!’ Depending upon the type of Subsoil, Foundations, and Local History (Informal Mining, etc.,) a remedy is not beyond the realms of possibility. In the past, I have seen Pressure Grouting used as an effective method of Ground Stabilisation. Any form of Ground Stabilisation is for an Expert Contractor, and not a Jobbing Builder! Should Demolition be the only solution, Stabilisation will still be required together with Load Testing, prior to construction.

    In this instance, the fact that the house is a BISF Type is irrelevant! The problem is ‘Subsidence’ which can occur to any house, irrespective of the form of construction. The Foundation Design of a BISF House is not different from any other.

    Respectfully, I also believe that you should not be dealing with these matters yourself. By dealing with the Insurer via a Surveyor, Architect, Engineer, or Insurance Assessor, you will gain a sense of detachment, and will be treated very differently.

    It should be remembered that whilst you say that “I have been dealing with my Insurers," it is more than likely that you are dealing with a ‘Loss Adjuster,’ who has been appointed by the Insurer to handle the claim. It is the Loss Adjuster’s job to ‘Minimise the Claim!’ – He is not there to do YOU any favours! Matters have probably become protracted further, as there will probably be two Insurers and two Loss Adjusters – One for each property. Just like dealing with two doctors, there will be two separate and contradictory opinions!

    Most Loss Adjusters will be paid on an Hourly Fee, which gives credence to the argument that both the Insurer and the Loss Adjuster are in no mood for an early settlement! A few hundred pounds paid in Loss Adjuster’s Fees can help defer losses running into hundreds of thousands of pounds! This has been particularly relevant over recent years during the depressed economy.

    The Insurance Ombudsman has reported a 12% increase in complaints against Insurers during 2012-2013, and 90% of (a sample) 260 Insurance Brokers, reported that claims were being handled more strictly than ever before.

    Personally, I consider that four years is totally unacceptable! At the outset of the Claim, it was evident that subsidence had occurred, and damage caused. Placing ‘Tell-Tale Markers’ and recording further movement, were never going to remedy the original problem! It is rather like recording the Blood Pressure of a ‘Dead Body!’

    In addition, you have been drawn into a situation where you cannot sell your house, and you have watched what residual value there is (literally) ‘Sink in to the Ground!’ In short, you have suffered loss – Both personal and financial. Perhaps it is time that you spoke to a Lawyer, rather than an Insurer, or Builder? – I might add, that I am none of these!

    Looking at this situation from the Insurer’s Perspective, we can see that they have taken a few hundred pounds in Premiums, yet are now facing a Total Loss! However, it is the Insurer who is in the ‘Business of Risk’ and not YOU! By prevaricating, they are limiting their exposure, in the vain hope that the problem will eventually ‘Go Away!’ – Or, that the ‘Night Fairies’ will make a special visit!

    It is also likely that the nature of a BISF House is ‘Frightening’ them too! – Especially if the only solution is Demolition and Rebuild. Again, they were happy to take the Premiums, but are now looking for a way out! The plain truth is that the BISF Design in 2014 is still viable; yet has to address current Building Regulations. The original Post WW2 Design, had sufficient foresight, allowing for Regional Variations, and Upgrading. However, its ‘Achilles Heel’ was one of ‘Design Life;’ sometimes reflected in the poor choice of materials and detail.

    It is perfectly reasonable to assume that a rebuild would take a maximum of 12 weeks, using an experienced contractor, and a good standard of documentation. Both Planning Permission, and Building Regulations Approval will have to be obtained beforehand. It is more than likely that your Policy also includes for Professional Services from an Architect and Engineer too, together with provision for Temporary Accommodation.

    However, and here comes the ‘Sting in the Tail!’ When you took out the original Policy, the cost of Rebuilding should have included for Demolition and Site Clearance, Accommodation Work, Works to adjoining or affected properties, (Neighbours) etc., Professional Fees, the Rebuild, together with any additional costs such as Temporary Accommodation, etc. It should be plain to see that the Rebuild Cost is no reflection of the Market Value, and neither is it linked to a ‘Build Cost per Square Foot.’ Whilst most Insurers use Standard Tables, each home has to be assessed in a very detached and pragmatic manner.

    The first job that a Loss Adjuster does, is to measure your house, and property. He will also assess any additional costs. In short, he prepares his own ‘Rebuild Valuation.’ Should you have insured for £150,000 and his Valuation is £200,000 then your Claim will be paid at only 75% of the total! Insurer’s do not mention this when the Policy is being taken out! In the case of a Rebuild, you could be working with a substantially reduced budget. Insult is added to injury, when you then have to pay VAT on the works, and then wait to claim it back!

    Here, the parallel can be drawn with Car Insurance. An Insurer might be aware that an Insured Vehicle has previously been a Total Loss. They still accept the Premiums, and provide Third Party Cover, but will not settle any further claims due to the history of that vehicle. You only find out what ‘Fully Comprehensive’ means, when you make a claim!

    From personal experience, it is only once your Insurers are aware that you are going ‘Legal,’ that they will begin to take some positive action! Should your property still be under Mortgage, you are duty bound to inform the Mortgagee. You should not view this negatively – The Lender has a Vested Interest in your property, and could also be in a ‘Negative Equity’ situation. Having the Mortgagee behind your claim, can help tremendously. As a final piece of advice, ‘If it is not in writing – It never happened!’

    Remedial Solution 1

    Essentially, the Structural Frames for each property were designed to be independent of each other. However, Both are tied to the Party Wall, and for other reasons of ‘Post War Economy’ you will probably find that the Roof Purlins also connect the two properties. The Party wall should be a Cavity Construction (75mm Block, 70 Cavity, and 75 Block), with the two leaves being tied together. Your Ownership Line runs through the centre of the wall. Should one leaf of the Party Wall be removed, the wall will become Structurally Unstable. With both leaves of the wall remaining (for stability), you will effectively be encroaching upon your neighbour’s ownership. Here a ‘Deed of Adjustment’ will be required, to be registered with the Land Title. Do not forget that the Legal Costs here, might be claimed from the Insurer.

    The foundations for the Party Wall, and the Floor Slab Design will be specific to the site, and locality. It is unlikely that the Party Wall will be built from the slab alone, as the whole slab length will require a Construction Joint at some point, in order to mitigate movement and shrinkage. This is possibly why the subsidence to your property is not so severe as your neighbour’s.

    In the Loft Space, only one of the two leaves of the Party Wall is taken through to the underside of the Roof Sheeting. The stability of this section of wall is maintained by the last Roof Truss. Again, there should be two skins of the wall for Stability and also for Weather Resistance, if the Gable is to be exposed, by the demolition of one side. It is worth checking to see if the Loft Party Wall is on your side or your neighbour’s side.

    Having said that both frames are separate, they do work together (Via the Party Wall) in terms of resistance to wind loading. The Free Gable Wall, and the First Bay of the Front and Rear Walls, to each property contain diagonal braces at the two corners, There is also Wind Bracing in the Roof, at each end of the Semi. Should one half of the block be demolished, then additional bracing will be required at the Party Wall Corners, and in the Roof Space. The Party Wall itself does not contribute anything to Wind Resistance.

    Remedial Solution 2

    Alternatively, the Party Wall can be demolished completely, provided that a Steel End Frame is installed, complete with bracing (the same as the Free End). This will obviously have the greatest impact on the house, and will also leave the end totally exposed as the work is carried out. There will be little alternative here, apart from vacating the house for the period of the works.


    Whichever solution you choose, your assessment should be based upon ‘Which remedy puts the Market Value, and Saleability back into your house?’ DO NOT go along with what is easiest for your Insurer, or his Loss Adjuster!

  6. Hi Roger, sorry to hear about your problems. This is probably a very unusual problem, as a BISF house is much lighter than a brick or stone house and therefore should be less prone to subsiding, but of course this probably makes things more difficult for you as there might be no previous examples.

    I’ve thought of a two other underlying problems that may be the cause. Is your house built on a slope or some other built-up land? Ours is built on a terrace that has been cut out of a hillside so the back of the house is below ground level (there is a narrow back patio and then a 1.5 m retaining wall) and the front is above original ground level on a terrace built up with material excavated from the back. Obviously over time built-up ground like that can settle and cause subsidence so I was very careful to look for signs of cracking in the concrete slab at the front but found nothing significant. Other reasons for built-up land can be infilling of old quarries etc.

    The other underlying cause of subsidence apart from the trees that Marc suggested is mining. Are you in an old mining area? Here in Bath another area of the city suffered subsidence from old stone mines until they were filled in a while back – holes opened in gardens, some houses were evacuated, weight limits were put on roads etc.

    The main problem I see with removing the house next door would be that a BISF house has diagonal bracing in the corners to keep them square. If you lost the house next door you’d have no bracing in the new corners. It could be put in, but whether it would be cost effective I’m not sure.


  7. Hello Roger,
    What an unfortunate situation your are in.
    This is one of the first times that I have heard of a BISF house suffering from substantial subsidence. I hear about plenty of traditional brick properties developing subsidence issues but this really is a pretty rare occurrence.

    Firstly, when any long standing building suddenly develops subsidence issues, there is often a root cause nearby. There can be any number of underlying reasons but two main ones springs to mind.
    One of the most common culprits is foundation damage caused by tree roots. Are there any substantial sized trees located near to your neighbours property?
    Tree roots can often extend out 2-3 times further than the canopy and cause damage to underlying drains and foundations, particularly on clay soils. The roots can penetrate and damage the foundation as well as drawing vast amounts of moisture out of the surrounding soil, causing the ground to shrink.
    The other most common cause is heavy rain followed by periods of drought etc on clay soils but this doesn’t answer your question.

    In my view the bisf house was and is designed structurally to be built as either a semi detached or terraced dwelling. The steel frame gains much of it strength from the surrounding cladding which helps to tie the building together. The main structural support stanchions are located at each corner of the building with insufficient frame support in the center of the house to become and end wall should one half be removed.
    There may well be a concrete block wall running through the middle of the two houses but should one half of the dual building be removed, this wall would have no additional support. There would need to be substantial steel work applied to the severed wall surface including cross members to make this building safe. Rigidity would also become a problem as the BISF house was never designed to be detached.
    Now I’m not a structural engineer but you would need to seek advice from one. A J Balfour are engineers well suited to this type of property and may be able to assist.

    With the associated costs involved with part demolition here, it may well be cheaper to undertake a rebuild and I would hope that insurance companies would foot this bill.

    Can you provide us with any images of the structural damage as they would be invaluable for reference purposes.

    Kind regards