building an extension...

hi all,
not really sure which place to put this thread but here goes...
i want to build an extension for £20k ( roughly)....
i have £13,000 in savings
i have £50k equity in my house

i would like to keep my savings as they are, but want to know the best way to finance the extension....?
loan? extend mortgage? credit card(s)?
ideally pay it off over 4 years, so obvioulsy want to get best interest rate i can....
how would you guys do it?

thanks!