Borrowing more on the mortgage

[lolawoo]

This is probably a daft question but how does this work? We have approx £25k left (7 years) on our current mortgage on a decent lifetime tracker rate. We want to borrow more to extend our property. I presume the lifetime tracker rate cannot be increased (either term or sum borrowed)? So the options are (a) get a second mortgage at a different rate for the extra borrowing, or (b) get a totally new mortgage to cover the £25k plus extra borrowing?

Yes. Regarding A a more common option is a secured loan plus your existing mortgage.

We took out a second mortgage with our current mortgage company, it's on the same bill but allowed us to keep our excellent tracker.

You need to work out what’s cheaper. Your great rate on the 25k might be wiped out by a large second mortgage account on a much higher rate.

We had to effectively add a new mortgage with the rates they had at the time. We couldn’t tag it to our existing deal.

You need to look for the better deal. Call your current lender and ask about their rates for a further advance. Then have a look round to see what deals are out there if you took out a new mortgage for the full lot. Remember that you'll need to factor in any early redemption penalties you may have.