Hello,
What a terrible situation but there is always a solution.
I think your priority is to get the house habitable in the size it is. This will be replumbing/wiring etc but you can get ahead of the game because you know the future work ahead so can wire with the extra loads accounted for. It sounds like the amount of work has increased since you started planning with the leak etc and so plans must change accordingly. Once you're in, you are saving a lot more money from not renting, and you can then address phase ii of extending.
I've not seen plans so this is just my understanding from what you've said above. If you want to bounce some ideas around in a no obligation, completely impartial way, feel free to get in touch via my website www.thekitchenladyuk.co.uk I am not an architect but I am a trained designer who knows how to make things WORK!
Good luck
Emily
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over hob
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