Money, money, money

[JJD1]

Hi all,

 

I am at the very early stages of planning a self-build. The site is in London and is extremely constrained, which means the design includes a heavy cantilever (image below). The site measures only 11x6m total. At the moment, I am considering a steel frame design (mostly as a result of the need to support the cantilever). I am yet to have a soil survey carried out but know that it is highly likely that there is made ground to approx. 3m, superficial deposits to 7m 3and London clay beyond. Structural engineers are advising CFA piles - if you have other suggestions, please let me know.

 

I'm trying to estimate costs to see if the project is feasible but I'm not sure if I've missed anything? I'd be grateful if those with experience could take a look and give me any feedback please? Some indicative costs have been provided by tradespeople and others are pure guestimates, so would appreciate your thoughts.

 

 

Thanks.

Costs Spreadsheet.xlsx

[nod]

I would go with steel and block 

Your obviously trying to keep the cost down 

I’m in the NW of England and find your costings to low 

London prices are going to be even higher 

Bathrooms and kitchen 

Windows Doors Garage doors can be expensive 

Kitchen and bathrooms also 

I would put a support under that corner Cantilever look great but can be expensive 

 

Only my take Sorry if it sounds a bit negative 

 

[PeterW]

I reckon your costings are about 100% out ... sounds harsh but you have a difficult site and finishing this for less than £1500/m is going to be a really tall order. Piles and steelwork are going to be easily double - what about services such as drainage and water ..?

Edited March 7, 2021 by PeterW
Typo..!!

[nod]

I would go with steel and block 

Your obviously trying to keep the cost down 

I’m in the NW of England and find your costings to low 

London prices are going to be even higher 

Bathrooms and kitchen 

Windows Doors Garage doors can be expensive 

Kitchen and bathrooms also 

I would put a support under that corner Cantilever look great but can be expensive 

 

Only my take Sorry if it sounds a bit negative 

 

Also scaffolding and crane to factor in 

 

[JJD1]

 

1 minute ago, nod said:

I would go with steel and block 

Your obviously trying to keep the cost down 

I’m in the NW of England and find your costings to low 

London prices are going to be even higher 

Bathrooms and kitchen 

Windows Doors Garage doors can be expensive 

Kitchen and bathrooms also 

I would put a support under that corner Cantilever look great but can be expensive 

 

Only my take Sorry if it sounds a bit negative 

 

Thanks for your feedback, nod. No, not negative in the slightest... I am new to the self-build world and in the very early stages of planning a project so appreciate your comments. I am trying to keep the cost down, but not to the extent of being unrealistic, so all feedback from experience very much welcome.

 

Bathrooms and kitchens I am content with and have experience in from previous renovations, so not too worried about those costs. It won't be anything particularly fancy.

 

I accept your point on windows and the door, but point to note that the garage doors are not mine (although I may end up re-providing them if the build so requires) - they are only to demonstrate the need for access of neighbours who have existing rights of way. 

 

I'd love to include a supporting column for the cantilever too, but site constraints mean that it's not looking likely to be possible. 

 

Cheers!

[JJD1]

3 minutes ago, nod said:

Also scaffolding and crane to factor in 

 

Just a minor point I am missing then... ?. How on earth do I estimate that!? Any ideas gratefully received?

[JJD1]

10 minutes ago, PeterW said:

I reckon your coatings are about 100% out ... sounds harsh but you have a difficult site and finishing this for less than £1500/m is going to be a really tall order. Piles and steelwork are going to be easily double - what about services such as drainage and water ..?

Thanks for the feedback, Peter.

 

By coatings, do you mean external rendering etc? Would you be able to expand on what might be involved so I can break it down please?

 

I had done some research on piles and looked at other comments on this site for CFA piles to guide an estimation on the cost in consideration of the fact that the section requiring piles is just 30sqm, but appreciate that there are a lot of variables and always trying to 'add a bit' for the fact it's in London.

 

I had factored connections for drainage, water and electric into Stage 1 and allocated £10k in the spreadsheet on line 8. The site sits on the roadside alongside other residential properties and the sewer runs along the middle of the road, so not far to go.

 

Thanks Peter.

[Gav_P]

I personally think you are easily 50% under, you prices are very sporty for a 4 story building even for someone in the trade with all the contacts,  discounts and experience that brings.  You may get to water tight for that budget. 
 

Did you price for the glass on the terrace (not sure if that was in your breakdown)... I’m guessing they are a big chunk of cash? 

 

Where do you put all your materials? For example when all your blocks or steels are delivered, where are they placed until they are used? Is there space for scaffolding or do you need a different solution? 
 

exciting time and quite a challenging site ? 

[Moonshine]

Sorry but £800 per m2 is dream land sort of money especially in London.

I suggest getting some drawings drafted and getting over to a QS to estimate costings

[JJD1]

3 minutes ago, Gav_P said:

I personally think you are easily 50% under, you prices are very sporty for a 4 story building even for someone in the trade with all the contacts,  discounts and experience that brings.  You may get to water tight for that budget. 
 

Did you price for the glass on the terrace (not sure if that was in your breakdown)... I’m guessing they are a big chunk of cash? 

 

Where do you put all your materials? For example when all your blocks or steels are delivered, where are they placed until they are used? Is there space for scaffolding or do you need a different solution? 
 

exciting time and quite a challenging site ? 

Thanks Gav. All good points and exactly why I'm posting this to those who have experience, so thanks for the feedback. Definitely exciting... if I can pull it off. Might be a few years off with your estimations though... ?

 

4 minutes ago, Moonshine said:

Sorry but £800 per m2 is dream land sort of money especially in London.

I suggest getting some drawings drafted and getting over to a QS to estimate costings

Cheers Moonshine. Absolutely will, but just trying to get a really early feel for whether it's feasible first in consideration of my finances. Posting this to see if I could get some feedback on the breakdown of costs to see where I am way out... and clearly I am in a lot of places.

[nod]

28 minutes ago, JJD1 said:

 

Just a minor point I am missing then... ?. How on earth do I estimate that!? Any ideas gratefully received?

Quick call will give you a good idea

Un my knack of the woods a crane about a £1000 per day Scaffolding 20m2 Though I would expect your location to be more 

[nod]

8 minutes ago, JJD1 said:

Thanks Gav. All good points and exactly why I'm posting this to those who have experience, so thanks for the feedback. Definitely exciting... if I can pull it off. Might be a few years off with your estimations though... ?

 

Cheers Moonshine. Absolutely will, but just trying to get a really early feel for whether it's feasible first in consideration of my finances. Posting this to see if I could get some feedback on the breakdown of costs to see where I am way out... and clearly I am in a lot of places.

We came in at 815m2 but had to do most ourselves 

and had the advantage of another 100m2 over you which helps spread the cost I would expect to add 40% to ours in the SE 

 

Though I do think you are over allowing of your professional fees 

Edited March 6, 2021 by nod

[PeterW]

1 hour ago, JJD1 said:

Thanks for the feedback, Peter.

 

By coatings, do you mean external rendering etc? Would you be able to expand on what might be involved so I can break it down please?

 

I had done some research on piles and looked at other comments on this site for CFA piles to guide an estimation on the cost in consideration of the fact that the section requiring piles is just 30sqm, but appreciate that there are a lot of variables and always trying to 'add a bit' for the fact it's in London.

 

I had factored connections for drainage, water and electric into Stage 1 and allocated £10k in the spreadsheet on line 8. The site sits on the roadside alongside other residential properties and the sewer runs along the middle of the road, so not far to go.

 

Thanks Peter.

Should say costings not coatings although I can’t see how much you’ve got in for that ..! Insulation and that sort of thing will chew £40/m so that could be £7-8k. 
 

Your issue with piling isn’t that you “only” need 30 sqm, it’s you need piles to stop the building rotating about its middle wall. You’ve got to basically load the cantilever to keep it from rotating and you’re going to need a huge spine wall to do it. That point load will need a lot support and you’ll need to ensure the building is balanced.  
 

All the steels will need to be very big too, wouldn’t surprise me if the structural cost was £60-80k on top of a £50k foundation cost. Then add in the walls, insulation and that could cost you another £30k with scaffold and access etc. 
 

@Gus Potter whats your thoughts ..? Feasible ..?

[Conor]

For a normal cantilever building using standard techniques, you want the anchoring wall to be double the length of the cantilever*. You don't have that. I'm not sure this building is constructable without extreme (expensive) structural solutions not normally used in standard residential construction. How much detail has your SE looked at?

 

At a guess, steel framed building with lightweight wall panels (sips) would be the obvious option to minimise the engineering required for the cantilever.

 

What's the constraint that allows to build over land as a cantilever but not put in posts/supports? A couple conctrete columns would knock several tens of thousands off the build cost.

 

 

 

 

* I am not a structural engineer, just a lowly environmental engineer ?

 

Edited March 7, 2021 by Conor

[Ronan 1]

I agree with Conor above, that looks to be almost unconstructable  without some support and I'd imagine it will add a 6 figure sum to your building cost if it is.

 

If you can get a few columns under it it would be a while different story and a completely different build.

 

Best of luck with it though.

Edited March 7, 2021 by Ronan 1
Because i cant spell and bloody auto correct is a pain

[ProDave]

What is actually stopping you having a pillar in that corner?  As long as the access is still wide enough?

[Gus Potter]

On 07/03/2021 at 00:51, PeterW said:

Should say costings not coatings although I can’t see how much you’ve got in for that ..! Insulation and that sort of thing will chew £40/m so that could be £7-8k. 
 

Your issue with piling isn’t that you “only” need 30 sqm, it’s you need piles to stop the building rotating about its middle wall. You’ve got to basically load the cantilever to keep it from rotating and you’re going to need a huge spine wall to do it. That point load will need a lot support and you’ll need to ensure the building is balanced.  
 

All the steels will need to be very big too, wouldn’t surprise me if the structural cost was £60-80k on top of a £50k foundation cost. Then add in the walls, insulation and that could cost you another £30k with scaffold and access etc. 
 

@Gus Potter whats your thoughts ..? Feasible ..?

Oh Peter you have put me on the spot.

 

Having a look at the section. Structurally big cantilever, more span than available back weight.

 

That bit that is over hanging is much more than the main width of the four storey part. For all, think of it like a swing.. pivoted just to the right of the door. If all the building is the same weight it will probably want to tip over under it's own weight.

 

But.. Peter. Could you Architecturally make the left hand side of the section heavy.. do this in brick / block, make the over hanging cantilever zinc external say.. not masonry.. say on a steel stud work, use a wriggly tin slab on the four storey bit.. you'll need a good floor thickness anyway for fire regs.. now you have ballast for the cantilever... for all, you now add a big weight to the short end of the swing so it won't fall over. It may be that you need to add a good mass to the left wall ( Trombe wall possibilty? ) using masonry..you will need a good bit though.

 

The side elevation looks good.. not too many openings. This leads me to thinking about a simple steel braced frame that fabricators churn out every day.. easy to insulate and get priced up.. simple steel connections and so on. A simple design that you can easily attract tenders.. cost effective.. go for simple stupid. But in some ways this is good economic design... best to spend the money on Architectural features than heavy steels /piles you hide?

 

One key would be to nail down the construction sequence so the thing does not fall down when you are putting it up. Provide enough back weight at all times so you go you don't need too design the left side piles as tension piles. Fine for a major client as tension piles ( still cost a bit more) are fine on occasion but as a self builder keep it simple  / stupid and you'll get more tender options.

 

I think it's just doable engineering wise for a domestic client at London prices, but.. they need to invest early to get a buildable design that leads the contractor by the hand. Do this and they may well get this in on or below budget. It's a must to get the Architect and SE working together from day one and in short order bring on board a trusted contactor who undestands the stability issues.

 

That's my first go at this! and  please excuse the spelling and grammer.

 

 

 

 

 

Edited March 9, 2021 by Gus Potter
claity, my left handed typos, is't social and enjoyable

[Russell griffiths]

What part of London. 

 

If you get that built for under £400,000 I will be very surprised. 

[markc]

@Gus Potter has nailed it from a structural and stability point of view

[Gus Potter]

On 09/03/2021 at 08:46, markc said:

@Gus Potter has nailed it from a structural and stability point of view

Thanks MarckC for the confidence. Maybe, but it's touch and go!

 

On 09/03/2021 at 07:47, Russell griffiths said:

What part of London. 

 

If you get that built for under £400,000 I will be very surprised. 

 Russell. Yes I think you are right to be cautious here with the budget.

 

Also Gav " I personally think you are easily 50% under, your prices are very sporty for a 4 story building.." That's a new expression for me, I like "sporty" pricing concept!

 

Pro Dave nails it too. If you can introduce some columns then you can make the cantilever problem go away and bring the cost down.

 

PeterW.. yes even up north here I would be looking at 2.0 - 2.5K per sqm as this looks like it deserves some attention to quality detail. But JJD1 says it's a constrained site so yes 3.0k /sq m given the possible foundation / building stability costs.

 

JJD1. A design like this could be stunning once you start to develop it up. You could do some great looking slick detail with the structure and really make a statement. It's a good SE challenge to do something like this on a tight budget. JJD1 ..It looks like you or your designer has considered the exposed slanting roof members over the terrace which would act as "cables". In some ways you have something like an asymmetric cabled stayed bridge concept here, but with less onerous loadings.

 

It may well be that at London property prices this could be viable? JJD1 can you post some elevations and a site plan?