Using existing foundations for a new build

[allthatpebbledash]

Presumably to @Iceverge much sought delight, I embark upon my next point of discussion here, should I go back to the idea of a full demolition and new build?

 

To understand the context of the post, you can read this topic here.

Okay so, I have a stripped out house that’s standing as a shell. Bricks, blocks and timbers with a roof on. Externally it’s covered with pebbledash which I want to get rid of. Insulation is poor in the cavity walls with none at all below ground floor.

 

Being empty for 2 years means my VAT is going to be 5% for this, so a huge saving there.

 

I am/was planning on retrofitting, so all new windows and doors, new ground and intermediate floor decks, removing the old roof and putting on a entire new roof structure, rewiring, new plumbing, improve insulation, essentially fitting it all out again.

 

The retrofit plans require demolition of all the rear wall and adding a new single storey extension and then rebuild the first floor wall, and there’s also demolition of half of the front house wall and adding a new double storey extension here.

 

This is now suggesting to me given everything else being taken out, I might as well do a full demolition and build a new house. Much better built, better insulated, more comfortable, better looking (externally), overall a slightly less of a headache. 
 

But, I have concerns over the cost.

 

The existing house floor area approx is;

145sqm GF

135sqm FF

280sqm TOTAL 

 

I understand from reading various topics on the forum, and elsewhere, I can estimate a build cost of £2000sqm.

 

With that in mind, I think a replacement 2 storey house is ideal. If possible, go into the loft for added floor space as I understand this won’t add much cost on to the build.

 

If built back to the same floor area, I’m looking at a construction and fit out cost of approx £560,000.

 

Can someone elaborate based on their experience and knowledge, firstly if I’ve reached a correct understanding of this, and if there’s ways to reduce costs?

 

Like for instance I already have services connected to the plot as there’s an existing house there, can I knock £25,000 or more off that figure?

 

The existing footprint is ideal for us, so drainage around the house can be reused in some or most parts? How much would that knock off?

 

And the BIG question here, given the existing footprint is ideal, can I reuse the existing foundations? How much would

this knock off that figure?

 

Obviously the challenge here would be for the architect to creatively fill the floor space within this.

 

If I also then went for a fuss-less finish, how much less can that per sqm estimate figure come down to, £1500sqm? Or even less, £1000sqm?

 

Let me hear your thoughts, advice, suggestions, experiences, all of it.

[twice round the block]

You'll have excavate the existing footings to see how deep they are, and whether they would meet current building regs and satisfy the architect 

[nod]

Wip them out 

You could do all the investigations and BC could turn up an say Nah Take them out Or ask for a SE report Who will probably advise the same thing 

[bmj1]

I think a silly corner to cut. Take out and start again.

[Iceverge]

11 hours ago, allthatpebbledash said:

can I reuse the existing foundations? How much would

 

Probably not. 

 

Maybe if you were to have a lighter superstructures like timber frame. 

 

Have you considered a basement? 

Edited Friday at 00:13 by Iceverge

[Iceverge]

11 hours ago, allthatpebbledash said:

Presumably to @Iceverge much sought delight

 

Delight is pushing it!! 

 

Given the labour and uncertainty involved in renovations it's a pragmatic decision. 

[Gus Potter]

I have a small project on the go. I'm replacing an existing conservatory and putting a solid structure on top and bringing that into the house insulation envelope.. but retaining what is a good solid base. The base (substructue and floor) is worth about 20k as it is elevated. The planners fought back but I played the sustainablity card.. as per the small print in thier guidance. That make the project viable.

 

@allthatpebbledash I think that for the extent of the work you are proposing retaining the existing founds will not be cost effective. One reason is that you may want some open plan spaces.. that often introduces point loads that the existing founds were never designed for.  Now you may need extra pads and tie all that in.. this is expensive labour wise not least.

 

 

[Benpointer]

10 hours ago, Gus Potter said:

[Snip]

@allthatpebbledash I think that for the extent of the work you are proposing retaining the existing founds will not be cost effective. One reason is that you may want some open plan spaces.. that often introduces point loads that the existing founds were never designed for.  Now you may need extra pads and tie all that in.. this is expensive labour wise not least.

 

 

You think the foundations for a 50s/60s bungalow were 'designed'?    

 

When we extended and refurbished a 1960 bungalow, having built a big, well-insulated extension we had intended to gut and radically improve the insulation of the original house.  But as we started to dig up the concrete floor to put in a new insulated floor we quickly found ourselves below the original foundations, which only seemed to be 500mm deep.  At that point discretion got the better of valour and we quickly repaired the floor, accepting sub-optimal flooring heat losses to avoid walls falling down or expensive under-pinning.

Edited Friday at 11:20 by Benpointer

[kandgmitchell]

Presumably you have planning permission and building regs for what is a substantial renovation and extension to an existing dwelling. Knock it all down and you will will now need planning approval and building regulations for a new dwelling. Depending on the planning status of the land viz a vis the local plan, getting a new dwelling approval may not be as simple and of course you'll have the delights of complying with all the building regulation requirements for a new dwelling which are more extensive than those for extensions.

[Gus Potter]

On 07/03/2025 at 11:19, Benpointer said:

You think the foundations for a 50s/60s bungalow were 'designed'?  

Good point! No I don't think many elements were "designed" in the modern sense where we may do extensive calculations to cover higher point loads and settlement for example.

 

Since man started putting up houses we have relied on tacit knowledge and practical experience. In other words we know what works.. but often it's almost impossible to prove by calculation. A 50- 60's house often relies on this knowledge base. Shallow founds can be quite common. They were building so many houses at the time that if the odd one suffered a bit from frost heave, near a tree it was not a major consideration. The important thing was to create employment ( East Kilbride new town is a good example) and to improve the living conditions of as many people as possible at the lowest cost.

 

Incidentally this lead to what we call non traditional housing. The Builders (George Wimpey, yes this was his name, etc) were experimenting with new alternative construction methods.. some worked well, others less so. Remember that at that time we were trying to improve the basic quality of the housing stock.. an inside bathroom was a luxury in many cases.

 

Another different case is in Victorian houses. The internal door frames are often made out of 6" x 2" timber, a goal post. From time to time if they had some extra load from above they would construct a shallow brick arch over the top of that (often called a relief arch) which acted compositly with the timber door frame. This is akin to a recent BH poster who asked about composite concrete lintels where the brick works together with a shallow prestressed concrete lintel.

 

As a designer I love working on old structures as the challenges and nuances are fascinating. Often I'll refer to my library of historic design information to support my design assumptions when say BC or a checking Engineer ask for a bit of validation.

 

On 06/03/2025 at 13:08, allthatpebbledash said:

should I go back to the idea of a full demolition and new build?

One simple way to approach this is to calculate the weight of the existing building and the stress on the soil under the existing foundation and try and make sure that the new building doesn't exceed this load. When it does you have to think whether further settlement will occur, how much and how the existing concrete foundations have aged (often 50- 60's house). On Victorian or earlier houses with say brick spread foundations or big stones just laid on the ground this can in some ways be easier.. as stone does not age.. but the lime mortar does to some extent.