Hullo Masklin, it does sound like a tricky situation, and I'm sorry to hear about these additional costs. Building work is always expensive to begin with, and additional and unexpected costs are never welcome. In respect of whether there is scope to obtain costs for this from your architect and structural engineer, this will most likely come down to the original appointment, their scope of involvement in the project, and whether they where direct appointments with you - or whether the SE was appointed through the architect. If the architect and SE were appointed to undertake planning and building regulations drawings with no exploratory or invasive works (ie stripping back existing structure in the house to assess structure before the project was tendered) then there will have been a number of reasonable assumptions on their part, which will only have been assessed when a main contractor started. The conversion of the attic would have added some additional loading to the property, but certainly less than adding a whole additional storey. There is every chance that additional bedroom space at this level would have less weight implication than someone stuffing their attic full with storage. SE's are notoriously cautious, so their assumption for the attic should have taken into account any additional loading, and if an existing (previously completed) opening needs to be reinforced, it might suggest that the existing opening was not designed appropriately to begin with. The flaws in existing structural design would generally fall into 'unforeseen' items, which is generally where contingency funds are intended for use. However, as others have suggested, if you are unhappy you can always pursue this further through the ARB, or RIBA (if your architect is a member of these bodies) or through the body that the structural engineer belongs too - but it is always worthwhile starting a discussion with the architect and SE and asking them to clarify how this happened. They will no doubt want to try and address this and have the opportunity to resolve this with you direct. Check your appointments with them, and what they were briefed (and paid) to undertake on your behalf. It might also be worthwhile trying to source any information you can about the original works and structural beam that got installed. Hopefully there will be a positive outcome from it all. Good luck! K :)

An architect designs buildings. A structural engineer works out the mathematics of how that building will stand. If you want someone to design the new part as well the existing part you need an architect. If there are structural changes the architect will hire a structural engineer to work out how those changes can be done. Once the design and structural calculations have been done the architect will put them together and submit the planning and building regs applications for you.

We used a structural engineer who was recommended by our builders. They had a relationship and it worked very well. At least if they know each other he will be more inclined to answer questions and help when needed. For a working SE this is a nuisance job in a lot of cases. Its a bit like working as a bathroom fitter/designer and then having to fix drippy taps. It sounds as though his assumption was based on the previous work being completed to a certain standard and maybe it wasn't. Did you receive any building control documents with regard to the work when you bought the house? We did not have existing steels when we took down a wall so ours was working with a clean slate so to speak. It sounds as though there are many factors at play here which complicates the job. You could go back to him and renegociate in light of the new discoveries. It seems a waste to pay for the same job twice. I would have expected a part drawing or sketch showing exactly where the steel was to be installed and padstone sizes etc. If you can do it without a lot of drama then I would go back to him and get him to revisit his calcs in light of what has been discovered and take it from there. If the relationship is now a bit broken rather find someone else. This is important you don't want to take any chances.

I would contact a few structural engineers first to see if any are interested or available to come and survey the problem and give you a quote. If it's only correcting the problem then I don't think you need an architect for this. Try and find a SE that does a lot of residential work as it's not that easy to find one. Agree about not getting involved in underpinning. This should have been done before either of you started renovation work. Your instincts are spot on. I would never use an SE recommended by a builder although I have done structural work on a previous house with great builders recommended by my SE. If the SE finds that the party wall needs underpinning it's worth having a conversation with your neighbours about it so at least they are aware of the building's movement.