My advice is somewhere between the two above.
Only you can assess the security of your job and if you genuinely think that you can take on a bigger mortage, having stress tested the amount carefully, and it is the right thing for your family, then you should go ahead.
However, you will have to convince a bank that your plan is financially sound.
You are asking for a second mortgage and it may not happen,
If the self build is your life long dream, you may have to sell up now and rent to fund it.