Extension - Idea of cost differences for steelwork options

[philnic]

We are planning an extension on the back of our house, there are two versions and I need to try to work out if there is a huge difference in cost, before I pay for the engineer and tech drawings for the option I plump for. Basically its a two storey extension out the back, and involves opening up on the ground floor.

Option 1 involves a + shape of steels with a central pillar where the corner of the existing house is. Bottom part of the + carries the old flank wall above for 5m, left part carries the old rear wall for 4m, top part of the + carries a new flank wall on 2nd storey for 2m, (and also has a steel supporting the new upstairs rear wall joining into it), and right part of the + carries an existing bit of single storey roof.

Option 2 is to remove the pillar. In order to do this the engineer has suggested a box frame of steel to do the bottom and top bits of the +, which is now a single span of 8m of steel, resting on steel columns, and I believe with another steel in the floor to complete the frame. The left and right bits of the + remain as in option 1 but bolt onto the box frame top beam rather than intersect where the pillar was in option 1. I should clarify that it might just be a goalpost.

Removing the pillar is great for the design, but not critical. If its going to cost us 2k extra over the version with the pillar its a no brainer, if its 200k we aren't doing it. Am sure its somewhere in between!

I get that we now have to manhandle one or two 8m pieces of steel onsite instead of smaller pieces, more footings presumably for the pillars at the ends of the frame, and obviously more physical steel to purchase, but I just don't know how much the impact of this and the extra labour to build it all would cost.

Anyone got a clue? Many thanks

 

[tony1851]

Sorry, haven't a clue what's going on with all the "+"s floating around; a rough sketch would help.

However, you mention a goalpost frame? These are expensive but not always necessary; why not consider letting the existing walls take some of the load instead of installing steel columns on their own separate pad foundations.

If your steels are to be 8m long, you can have them in shorter lengths with steel connecting plates drilled and ready for bolting together insitu.

 

[^woody^]

Looks like your spending a quite a few thousand on some grand design and are worried about a very small amount of money relative to the whole cost.

Here is a tip. As you're paying, you tell the engineer what you want, and if that's a cheap job with massive visible beams and piers say that, or if it's flush ceilings and walls tell him that.