What would you do in this situation?

The first point to acknowledge is that everyone makes mistakes (both in the field of town planning and in every other area of life and work) - I've worked in town planning for 26 years now and man times I've come across this kind of scenario, or similar scenarios whereby Councils or other statutory bodies have made factual errors. I'm currently managing an application for over 70 homes which is now being extensively delayed/having to be redesigned because when the client's flood risk consultant contacted the Environment Agency to obtain modelling data, it transpires the EA provided the wrong data. Even I've made the odd mistake - no-one is perfect.

Whether the 'engineer' is a sole trader or part of a large multidisciplinary consultancy, if they are calling themselves an architect or a town planner then to do so they must be either a Member of RIBA (architect) or RTPI (planner). But my (RTPI) code of conduct doesn't say I can't ever make a mistake, it says that Members are expected "to apply reasonable standards of skill, knowledge and care in the performance of all their work" so what is defined as 'reasonable' is open to interpretation.

Nothing in my written terms of engagement says if I make a mistake the client is due compensation or a refund, and I never work on a no win no fee basis. Similarly many people working in these fields are not chartered members of their respective professional bodies (I am, but quite frankly I feel I get no benefit from it and every year I have to pay a considerable annual membership fee).

But, in saying all of the above, I often say that the test of any professional is how they conduct themselves when things go wrong. I believe in holding my hands up, taking responsibility and seeing if/how I can resolve the issue, but plenty of other people will try to cover up their mistakes or deflect blame elsewhere. From what the OP has posted the behaviour of the engineer post-submission suggests they were very much in the latter camp.

It amazes me what my fellow 'professionals' get away with. Recently I was approached by someone who had got someone calling them an 'architect' to submit what needed to be quite a robust application (in terms of the amount of supporting reports). The client had been forwarded the automated confirmation that the application had been submitted....and 10 months later hadn't heard again from this 'architect' despite multiple emails etc.

The client hadn't done any digging themselves so rang me up (I was recommended) and when I looked into what had happened it transpired the application hadn't been validated because lots of supporting information was missing. I asked the client if he could forward me a copy of what the 'architect' had submitted - "I don't have that, the guy didn't ever send me anything !". So putting 2 and 2 together this character had probably submitted little more than an application form (but didn't want the client to know that), pocketed the professional fees paid by the client, and then disappeared.

But back to the OP's issue, as others have said it could be that a relatively minor change to the size/positioning of the proposed extension could make it permitted development. It's not so much the technical mistake I'd have the issue with, it's the behaviour post-submission.

If I was the OP I'd try to work out what changes I'd need to make to the proposed extension to satisfy the technical criteria. I'd then write to the engineer setting out my unhappiness and asking what action they are willing to take. Having a complete falling put/cessation of relationship with the engineer may be more financially costly to the OP, as the OP then has to find someone else.

If I was the engineer if this situation I'd want to make up for my error by working with the OP to design an extension that satisfies the technical criteria (assuming that extension is then still worth pursuing in terms of the benefit to the OP), and to resubmit that application without any additional charge and to pay any additional application fees out of my back pocket. But I'm not the engineer, and it may well be that, like the 'architect' character I mentioned, he's just disappeared into the sunset.

But the OP should try to have that conversation, initially adopting a 'mistakes happen, how can we go forward ?" tone, and see how the engineer responds.

The fact that you got this person to do the work suggests that you want to get this extension done.

It has failed the permitted development criteria.

However, you have not gone through the planning application process for an extension which does not meet permitted development criteria.

So, if I was in your shoes, I would apply through the normal planning permission process. This would require approval from your local planning authority and they would seek comments from neighbours.

On a positive note, you have discovered that there have been no objections (so far) from neighbours.

There is an interesint post about this on reddit today

reddit.com/r/H…rue (edited)

To be fair, an engineer is not someone I would be contracting to put forward a planning application, whether or not it falls within Permitted Development.

I would send a formal letter of complaint to his company. When you say engineer - a surveyor or architect would normally draw up the planning application details I would have thought. Presumably he is a member of a professional body so it would be worth writing to them as well. He should have professional indemnity insurance.