Building close to existing shrubs

owd potter

I am planning to build walls and create raised beds around what will be a new parking area on what I loosely describe as my front lawn.
There are several mature shrubs that I wish to retain which are located close to the line on which I intend to build a new retaining wall, and so will need to work around.
The plants that are likely to be affected are a 2.5m high Lilac, a 1.80m high deciduous Azalea and a 1.50m high Rhododendron.
In order that I can finalise the position of the wall (my current plan is shown by stringline on pic below), and before I start digging, it would help to know how large is the root ball on these plants likely to be and how tolerant are they to root disturbance / partial loss if I have to cut any back?
TIA
Owd

Lizzie27

Looking at your string line, I think the two smaller shrubs are likely to be okay but the lilac on the left will have longer roots under your lawn which you will probably have to cut. It might or might not survive this, but lilacs are easily replaced and grow quickly so I wouldn't worry too much.

BobTheGardener

I think the string line is far enough away from them all.  From my experience, the worst that could happen is that the lilac might start sending up suckers if any large roots are severed but those can be pulled-up.  Remember that cement is highly alkaline so the most important thing is to use minimal amounts and clear up debris as you go to lessen any effect on the two acid lovers.  I would cover any setting concrete/cement with plastic sheeting until it dries to eliminate any run-off if it rains.

owd potter

Thanks @Lizzie27
The delineated area is destined to become steps, descending from the peg at the foot of the lilac toward the car.
Depth of dig will also play a part here and at the top (adjacent Lilac), it will be approx 300mm increasing to 1.50m at the bottom. 
Consequently, I think, despite being closest to the line, the Lilac may be the least affected but I expect I will need to trim/remove some shallower roots.
It is the other plants that I am more concerned about (the centre line of which will be about 800m from my excavation line, and hence my query about how tolerant these plants are to disturbance.
I would hate to lose either. 

owd potter

thanks @BobTheGardener
Good reminders.

owd potter

thanks @Rik56
no services here...
yet.

owd potter

Dodging between showers, today I started stripping turf from the lower wall and steps area and found these dozing beneath...
They are in the bird feeder now.
Anyone? 

debs64

Chafer grubs? I think it’s the larvae of crane flies. Quite common in lawns I think but the birds will enjoy them 

owd potter

Thanks @debs64
the stuff you learn..
I love this place.

Hostafan1

 I can't see any harm coming to any of those plants at that distance. Remember the roots radiate  360 degrees . I'd not worry in the slightest.

owd potter

Thanks @Hostafan1 .
In the above pic, what is now turf stripped is where I'd really like to position the steps, and which would mean me installing a wall to the left of the steps as viewed (about where the shaft of my edging blade is). In this position the wall would be approx 800mm from the Rhodo & Azalea. Just not sure whether this is too close. Alternatively I may install the wall along the stringline with the steps to the right of it. 
Easier to achieve but I will lose some parking width which I am trying to maximise. 
I'd really like to get as close as possible without causing any adverse effects on the plants.
Decisions, decisions...