The trouble with this sort of site is that there will be so much rubbish and unwanted vegetation that you cannot see the wood for the trees. The best way of dealing with it is to consider what is already there and decide the best method of treating it.
Trees: Some of these might be quite useful in forming the framework of the new design, but it is important to find out what they are. Even if they are quite small at the time you take over the plot, they could be saplings of very large forest trees which will eventually become a nuisance. Their leaves will give you, and your neighbours, an endless autumn job, clearing them from lawns, paths and gutters. Leaves left to rot on a lawn will ruin it. Leaves on paths make them dangerously slippery. Leaves in gutters will eventually block downpipes and soakaways, making the walls of the property damp.
Leaves are not the only problem. Roots of big trees spread a considerable distance and can damage foundations, cause settlement and block drains. As a very rough rule of thumb, the roots of a tree spread at least as far in all directions as it is tall. For example, a tree 40 ft tall (just over 12 m) can have a root system of more than 40 ft radius (affecting over 5,000 sq ft or 450 sq m of ground), so it should be at least 40 ft away from any permanent structure. There are many exceptions to this rule, but it is a good guide until you become more familiar with the various species.
A third problem with unsuitable trees is the shade they cast, and the dry area under the spread of their branches. This can become annoying if house light is affected. Also it cuts down quite considerably the types of plant you can grow as those which appreciate dry, shady, impoverished conditions are limited. Remember that this can affect your neighbours just as much as it can affect you if the tree is near their boundary.
Observe also how healthy the trees are, and whether they are well shaped. There is no point planning a garden round an ailing, straggly specimen. However, if it has only small dead and dying pieces, these can be removed — otherwise it is better to lose it altogether.
One word of caution — many mature trees are subject to tree preservation orders, especially in conservation areas or where many properties are ‘listed buildings’. If you intend to do anything drastic, check with your local council first. If a listed tree creates certain problems, they will always meet you to discuss them, but if you remove a listed tree without permission, you cannot put it back again!