We got a mortgage.
But you need to speak to your bank about getting a mortgage for a self build, as I dont know how it works in the UK, as we are in Norway. It is quite common to build your own home here.
The budgeting and costing it the hardest thing. You need to read the specs closely, and your contract, and find out what the builder is contracted to actually supply. With many such builds you will find that only the most basic things are covered, and to complete it to your desired finnish will cost a lot more.
Like, quoting with vinyl flooring all over the house, and you deciding later you wanted oak floors, etc. Wall paper in bathroom, but then you are shown beautiful tiles. All these extras will add up.
I had a massive spreadsheet! When I decided we wanted beech flooring across the sitting room and kitchens, etc, I had to lower the specs for the bathroom units, and cost down the price for bathroom tiles, to ensure I stayed within the budget.
The worst thing, is when you go over budget, and you cant raise the money required to finnish your build (see, I have learnt a lot from Grand Designs. )
We also have 80% windows on the second floor (which is living space, our house is "upside down" according to British standards) and as we are in the arctic, our heating bills are large, as you can imagine!
But it is lovely. I love the smell of the house, and I love that I have seen it grow from just designs on a page, to a family home.