Property developing - my next step

[Dilan]

Up until now, my only experience of property developing has been doing up a flat, extending and renovating a Victorian cottage, and lastly converting a 1930's bungalow into a house. All of these have been pretty hands-on and have taken a bit of a toll on family life, as we've been living in the properties as we've worked on them.
Because of this, and as competition for "renovation projects" is getting to the point where there's little or no profit in it for me, I've decided to try something different.

I have found a row of 20 semi-detached houses with gardens about 100 - 120ft long, which back on to an existing road. These properties are, I think, all ex-Local Authority. Given the surrounding area, I reckon you could comfortably get 30 terraced / semi-detached 2 and 3 bedroom houses in these gardens, facing on to the existing road.

Assuming that these would sell for £180,000 to £210,000 each, and assuming build costs would be around £80,000 per unit (90m2 x £900/m2 - is this realistic?) each plot would be "worth" about £100,000, i.e. £3,000,000 in total. This "values" each garden at £150,000.

Given that I want to make a profit, and the developer I'd sell the land to needs to make a profit, what do you think is a reasonable starting offer, and maximum offer, for each garden?

How would you go about securing the land? I don't have the capital to offer to buy each one outright, and wouldn't want to in case planning was not forthcoming. I thought getting each owner to sign an Option Agreement, and giving them an "advance" of say £250 each as a sweetener, whilst I try to get OPP for the site and find a buyer.

If I get OPP, will I have to sell at a below-market rate as I will be selling 20 Option Agreements, rather than one vacant piece of land? If so, how much less?

I'd really appreciate any input from someone who's done something like this, as it's something completely new to me.

Thanks in advance