。
I want to build a new house next year.
Literal
Next-year [topic-は] new house [object-を] build-want.
建てたい is the desiderative ~たい on 建てる ('to build'). Note that ~たい inflects like an i-adjective (建てたい → 建てたかった past, 建てたくない negative). The は after 来年 marks 'next year' as the topic frame; the speaker is implicit.