i am currently bumbling my way through as i am doing my first townhouse development now - although have done some subdivisions before.
basically i learn an area and know what streets are in what zoning (sometime i need to look at the map again to refresh myself). our council have the zoning maps on the internet, but you can buy them from council as well.
so i look at anything in "district centre" - high density - over 500m2 ... over 500m is big enough for 3 2bedders. preferably over 10m wide for turning circles and flat.
when i find something of interest i contact the agent and go for a drive past. i then contact the council to see if there are any restrictions on the proprety like heritage listing etc. if all stacks up by them i recontact the agent to get a copy of the contract that will contain sewer plans, additional zoning restriction etc - you don't want a sewer line running thru the middle of the block!
then if the figures stack up - what the townhouses would be worth at the end, less building cost, less profit = what i'll offer for block - and only then will i make an offer. and not always. sometimes a block is only worth $200k to me and the vendor is asking $350k as a ppor - we're to far apart to even start negotiating, so i move onto the next one.
rang about a block yesterday but it is on a main, main road - so didn't even bother going for a look.
once i've bought the block then i am guided by the professionals - surveyors, architects, builders etc - they tell me what they need me to do.
hope this helps. ask any questions if anything is unclear.