bambam
where are you looking at sites?
i think the idea of a "brown-field" site is well worth investigating in the current market - it has become very desirable in certain urban areas in the uk and europe - but there are a few issues to consider -
build cost (at this early stage) could be budgeted anywhere between €2000-€3000 per sqm for medium spec and fit out.
assuming some sort of self-build mortgage, persuading your lender to come up with the funds wont be easy - they are looking for a lot more info than in the past and if you finished product is very different from the surrounding area it may be hard to put a future value on it.
unless the existing dwelling is totally delapidated you will be paying over the odds than for site only. especially if there is a recognised development potential (not as much as 2 years ago but still a premium)
demolition and site clearance can have significant costs and overheads,eg disposal of construction waste is not cheap nowadays in urban areas
upgrading existing services in an area that has been developed for a few decades can also be expensive
planning permission may not be straightforward for one-off housing in developed area, adjoining neighbours etc - however planning policy generally is supposed to promote use of brown-field sites
making an offer to a vendor, without it being subject to planning is risky but waiting 4 months or so may not be an option
professional fees etc may be higher if the job is priced and tendered as a single project to include demolition etc and your consultants charge a percentage fee.
however there is more than one way to skin a cat and if you find the right site, get some good advice and do your sums you could do something special and revitalise an old site. interested to hear how you get on!
m