Structural work vs bigger extension £££

[Jonny DIY]

Hi all, we’ve recently had planning permission approved and now trying to finalise our internal layout before getting in builders for quote. Budget is going to be tight, so one of the key considerations is weighing up whether to do more structural works or make the extension bigger, and looking to understand whether either is likely to be significantly more expensive than the other…

We have permission to go out 4.5m (on a 5.5m width) and this would be the preference (Option 1 below), but budget is a major factor and in theory for what we want to achieve (7.5m x 5.5m open plan space) we could get away with only needing to extend out 3m by using some of original house space, but that would require some significant structural works in taking out pretty much the whole 5.5m wide structural back wall (possible box frame/goalpost structure needed) - Option 2 below.

So looking for advice (if it’s a simple-ish answer) whether it’s likely to cost significantly more to do a bigger extension with less structural work, or to do the smaller extension but with more structural work? Or is it not that simple to answer and a case of needing to get the builders to quote for both designs?

In my (relatively inexperienced) mind, once we’re already extending at least 3m it feels like it would be more cost efficient & less complicated to just go out another +1.5m rather than having to go down the more “complex” structural route??

Simplistic drawings below to illustrate this particular scenario (mine, not the architects drawings, in case you weren’t sure )...

 

[Notch7]

Let’s hazard a guess

Structural wall opening £5k
Additional 8 m2….not sure but maybe a builder will put in a number here

My guess is the additional 8m2 will cost more, but you get more for your money

 

[^woody^]

If you've had planning permission approval, how are you going to make the extension bigger?

Let alone finalising the internals after applying.

 

[Jonny DIY]

Let’s hazard a guess

Structural wall opening £5k
Additional 8 m2….not sure but maybe a builder will put in a number here

My guess is the additional 8m2 will cost more, but you get more for your money

Thanks Notch... assume the £5k might be more for a "simple" opening? I got the impression from architect a goal post/box frame type structure would be needed given the span and as taking out the full wall, which might be significantly more?

If you've had planning permission approval, how are you going to make the extension bigger?

Let alone finalising the internals after applying.

The planning covers the full 4.5m extension, I'm suggesting an alternate could be to actually do less than we have permission for (understand this is fairly common when people find out actual costs!). And I was advised planning only really covers outside, how you layout interior is upto you (subject to SE calcs and Building Control which we've not yet proceeded on until we finalise layout)?

 

[^woody^]

You've been badly advised.

An extension is designed based on what you require or need, and this includes the size, structure, alterations, locations of doors and windows etc and the internal layout - you don't want to be putting the furniture in front of the new door openings for example. Then the permission is sought. The costs are taken in to account before all this, as its pointless designing a building that the occupants don't need or can't afford to build

You don't apply for a random extension then work this out afterwards. If you employed a designer, then I'd be looking for a refund.

Any alteration to a planning permission, requires either approval or confirmation that further approval is not required - even if building it smaller. Many planning depts charge for this.

 

[Murdochcat]

How much have you got in your contingency fund?

Time and time again I come across people with unfinished projects because they didn’t budget well

We had plans, started the foundations and discovered an unmapped surface water drain where our piles were supposed to go.

A few months later and with new foundations designed , costed and approved our 20% contingency was spent.

So we started our extension knowing that fitting out the shower room, the utility, the patio and drive would have to wait

 

[Notch7]

Thanks Notch... assume the £5k might be more for a "simple" opening? I got the impression from architect a goal post/box frame type structure would be needed given the span and as taking out the full wall, which might be significantly more

Yes you could well be correct, I haven’t pricing since before covid so prices are probably double.

A box frame is a fair bit of work as it means foundations and pads
And if you want a flush ceiling then replacing floors upstairs

Ultimately you will need to get a good builder on board who will give you the options…but you kind of need building regs drawings to get a builder to quote and you don’t want to get drawings done until you know how much each option will be. Chicken and egg

 

[Jonny DIY]

Thanks Notch... you see my issue exactly ha!

I guess my question is for builders then... If you're already onsite and building the extension of 5.5m x 3m at a rough quoted cost of £2.5k per sqm, then to take the extension out an extra 1.5m (to 5.5m x 4.5m) i.e. an extra 8 sqm, would I really be looking at £20k extra, or would there be economies of scale and the incremental cost of those 8 sqm be less than £2.5k per sqm??

Murdo, I'm hoping to have something in the region of 15%-20% contingency fund on the project cost.