Borrowing large amounts on credit cards

There are some dangers attached to this. 40K is certainly an achievable amount to get via credit card 0% deals, but you will be spread over several cards and be unlikely to be able to get more 0% credit cards at the end of the offer period (say 6-9 months for most of the deals) as you'll most likely be somewhere near the limit of what a lender will consider overcommitted.

Note also that if you were then to apply for a traditional fixed rate loan with 40K of CC debt on your file, you would have great difficulty getting accepted, so you may well end up stuck with it all on cards at very high rates and nowhere to go. You do also have to bear in mind that rates appear to be on the up, so your best chance of a decent long term fixed loan may be now rather than in 12 months.

Whether it's worth proceeding on the CC route will depend very much on how you intend to repay the debt and when you can do this. If you are transferring from a loan to a mortgage when the property redevelopment is completed, and anticipate this will be somewhere close to the end of the offer period for the shortest deal you are going for, then you may be OK (also assuming you don't have a large loan to value ratio on your house). The fact that the mortgage is secured on property and will be going towards clearing the CC debt will probably make it acceptable to a mortgage lender.

If you have no guarantee you will get the 40K back in (say) 9 months, i.e. you need to sell the results of your work (for example) or have some reliance on the equity of your house (which may go down), then I personally wouldn't use cards for this level of loan. I would look for a secured loan or flexible mortgage deal at a lowish rate and trade a bit of cash for certainty. I'd probably look at an offset style mortgage which could then be filled up with cash from credit cards where possible, rather than basing the whole strategy on credit cards from the outset.

Credit card 0% rates work best when they create no net debt, i.e. you save the cash somewhere else and use it to earn interest (or offset a mortgage). It's imperative that you can find some means of paying off the balance when you need to, as you can never absolutely guarantee what will happen with the next deal. My view is that you should always err on the side of caution, particularly when serious amounts are involved, as a mistake will be disasterous.

Just to be clear, I'm not saying it's not worth using 0% offers at all, but rather that it will be better to use a mortgage and offset that with anything you can get from 0% deals (hopefully the full 40K or even more, which shouldn't be difficult if there are two of you with decent incomes).

It's really swinging the deal around so that instead of using the CCs as the primary loan source, you take advantage of the fact that 0% deals exist to reduce your mortgage payments, but only after you've secured your position with the mortgage. This is much safer, as you either win or you avoid losing at all. You can also look to shift some of the debt onto low lifetime of balance offers too if you like.

It would seem to me that a flexible offset mortgage would be ideal for this, but you would need to look at your total mortgage amount (over and above the new 40k) to see if the offset is worth any penalty in terms of a higher interest rate. The alternative is to use the best mortgage rate and stooze your CC funds away at the best savings rate which may be better overall even taking tax into account - you will need to do some sums.