are you in the UK? just asking because of the time of your post. if you are in the UK, you can do this but you will need a solicitor. also, if you have a mortgage you may need to re-mortgage to do this.
I take it you are willing to just walk away but dont forget you may be intitled to some equity (if there is any - there may be a loss!). This depends on what your contribution to the relationship, financial or otherwise. It may be less hassle to walk away but you may wish see a solicitor. And be aware that fees nay be more than you gain, unless the bf is willing to pay all costs. Solicitors normally give one free consultation.
Quite right Ethel. You are entitled to half the rise in value but are also liable for half of any depreciation. Depends how much is at stake. It may not be worth the hassle.