I am aware that everything is subject to site conditions, and the quality of the original building, however we’d like to build out the back of the house to make a dining kitchen / lounge.

As far as I can see, we have 2 options, but its difficult to determine which is the cheaper. The house is a 1930s semi, about 5 metres wide. A 3 meter wide by 2 metre deep extension has already been put on to the back of the lounge.

Option 1 = demolish / re-use existing extension and put a 5 metre wide by 3 metre deep (overall when finished) extension on the back giving me a large L shaped space and the original kitchen to be used as a utility. AFAIK no strucutral work required. Would it be fair to say that the only real cost to consider is a new roof for my existing extension and disposal of new walls? Therefore I’m looking at 9m2 of new extension, plus 6m2 of sloping roof?

Option 2 = Add a small extension (2×2) to the back of the kitcehn so that it matches the existing extension, then knock through internal walls to make one large square space. Lots of structural work, leaving a column about 1/3 of the way across may make things simpler (on to which beams can be supported) and could form part of the kitchen design.

I’m thinking option 1 is the best plan as it gives me the most space in the end, and the potential unknown cost of all the structural work for option 2 and risks make the cost differences almost negligible?

I would say depends on quality/age of existing extension. If you demolish you will have to rebuild the new bit to building regs including the floor.

Didn’t realise the floor would be to building regs!

I think the extension was done mid 90s, but sounds like it would be best to assume that it would need re-building. I’ve crawled about under the floor space and it looks very well made, even has girders and stuff, but I’m not a builder!

OK, if its a suspended floor (which it is if you’ve been looking under it!) then much easier to insulate to current regs as insulation can go between the joists.

Everything has to be built to building regs. If you are building new walls, beyond the line of the existing, you will need foundations which again are subject to an inspection.

Option 1 definitely sounds best, as the alternative sounds messy and will not help you recoup your outlay when eventually selling.

Taking walls out and putting RSJ in etc is very straightforward, so not really a lot of risk and a builder can probably quote just looking at it as they’ll have done 100s of them….

I wish my extension was nice and simple like that! It’s going to involve 4 RSJ,s and a new retaining wall for the patio as I’m on a sloping plot. My builder mate scratches his head a log when looking at it!

I’d say there is little difference in cost and effort so go for the best result which will be 1. We’ve done a similar extension on our similar 1930s house. I did no structural changes to the existing house and kept pretty much all work to the new extension. That meant almost no upheaval to daily life. The old kitchen stayed functional the whole time. No issues with living without hot water or an oven. We now have a big kitchen diner and the old kitchen has become a good sized utility (handy as biker) and a downstairs loo. I was also pretty keen to minimise cost and environmental impact so we had no skips. Demolition waste was broken up on site and now forms the sub-base to the patio and my shed.