As a chartered Quantity Surveyor (yep I'm the boring git at parties) and working for one of the largest construction consultancies in the UK, we've had an internal email asking us to be more diligent on our client's construction projects to watch for signs of impending insolvency on contractors and sub-contractors. Seems we're bracing for more contractors to go bust. Hate to use the cliche, but we're in a perfect storm of rising costs of materials due to brexit, the pandemic and general shortage of labour, this is putting huge pressures on existing projects and afflicting new projects. Admittedly we work on large scale projects but this does still affect domestic builders also.
So my advice is make sure you do enough due diligence on any builder you intend to use, or get the designer, architect or PM company (if you engage the services of one, which I personally would as it does give you some peace of mind) to carry out due diligence to ensure solvency of your builders. Be wary of the lowest cost, especially if its significantly lower than others and any builder suggesting they could start immediately as they are normally warning signs (not always but any agency working on your behalf should be looking for this anyhow).
If using a agency (designer or architect and/or project management consultancy), get them to create a schedule of work based on their design and get the builders to price off that. It means that if you do need to adjust the contract price you can target specific items, if you don't and take a lump sum approach based off drawings without a detail schedule of works then you could be at the mercy of the builder when negotiating cost reductions/changes without a decent breakdown of their price. I used an agency that specialises in domestic extensions or refurbishments that dealt with the design, planning application, procurement and project management of the whole process which I paid maybe £1.5k max (to be fair I did use them as they were disability specialist and I needed to ensure the extension was adapted enough for my wheelchair bound son and we had the benefit of discounted rates for materials and the like).
Also make sure you have a contingency pot of cash for any unknown risks - most of the risk is in the substructure, if you're digging down for foundations there is a reasonable chance you might come across problems (once you're out of the ground the risks are much less), we did on our kitchen extension, our road's storm and foul drains run in our back gardens (don't know why) and the run was not even straight, they ran diagonally from my neighbour on the right to my neighbour on the left, and directly across where our foundations were going. We had to re-route the drainage (with the right permissions and checks from the Council) as we couldn't build over the drainage runs. There was no manhole covers to tell us this or even existing schematics, we had some idea that they could be there but we only found them once the builder started excavating the ground.
As your not looking to move its unlikely to be an issue, but if you're on a relatively new estate check your deeds to see if there is a need to get permission from the original developer (I had this on my kitchen extension a few years ago) you can ignore it but you would need to take out insurance if selling the house in later years. And keep everything safe (planning approval consent, all test certificates and completion certificates) once all finished.
And I agree on the timescales - easy 18-24 month process.
And good luck - its hard work, painful on the pocket and very stressful, however once its all done I'm sure you'll be more than happy and enjoy the extra space.
