If you intend cutting back your conifers and leaving them 10ft high and planting laurels nearby, then the newly-planted laurels will have to compete for light, moisture and nutrients with your conifers. As you are going to plant laurels that are 4ft high, then I would be inclined to wait until the spring and remove the existing conifers, roots and all and dig in as much moisture-retaining compost as possible. Yes, at first it will be very open but strong laurels that are of a decent size will motor away in the spring and put on a fair growth in the first year without the competition.

Nothing to add to that. In our next village someone cut back an established hedge and replanted without taking the roots out and I noticed today a trailer loaded up with dead new saplings. Frustrating because I was chatting to him last year and warned him he would need to take the roots out but he didn't listen.

Indeed. Very few things will establish under a conifer hedge or where a conifer hedge has been unless the roots are removed and plenty of new soil/fertiliser added to the area.

I have found that laurel grows surprisingly quickly to form a hedge.

Three 30 foot Conifers at the end of my Garden were property of the Council. After a waiting 2 years they finally paid Contractors to cut them down level with the soil. I was going to put Tree Root Killer on the bases, but the man told me that Conifer roots die when they are cut down. So maybe you would be best to cut them down level with the soil, then plant the Laurel. Chain saws can be hired for this job.
The Patio Roses I have planted 6 feet from the Conifer roots have thrived. No trouble at all.
The Laurels would bush out and cover this space.

i cant add to the first comment but as realy i think you should completly lose the conifers roots and all as youl need to replennish the soil as the connifers will of taken it all . an idea would be to put up some trellis 6` tall or make your own taller with battening from a builders merchants as its reasnably cheap. and grow a realy fast climber like virginia creeper or a few . you could take out a few trees at a time completly . dig in some compost etc and get your climber started and plant your young laurels and work your way back to your house perhaps . this way youl break the job down into managable amounts and your climbers will start growing realy strongly after a couple of years . then your laurals will start growing . i dont think there is a way of keeping complete privacy .i guess you could buy a 6` x 6` cheap fence panel and tack it to the trelles untill you dig the next couple of trees out . just a thaught .

"but the man told me that Conifer roots die when they are cut down. "

He is correct but it will take a long time and the soil around the roots will be totally depleted of nutrients

One of those jobs that needs doing in immediate stages?! Also, how long the hedge is may affect your decision?! 20ft high but how long/wide?

You will find it difficult to plant anything between the conifer roots if you only intend to take the tops out of the conifer hedge and plant laurel underneath. It is true that conifers will not sprout from their base cut. I suggest you first remove the conifer hedge, down to ground level, then stump grind all the root stumps and side roots out. THEN re-plant the line with laurel. You will, briefly, lose some privacy, until the laurel fills the gap, but the job will be done and finished. Temporary wicker/bamboo screening can be used until the laurel is grown.

Nosey Potter's comments only just come through to my computer ... I live in the stone-age! Anyway, I think NP and I are on same wave length about removal and re-planting.

I may of mis-lead some of you as I don`t plan to plant the Laurel hedge between each of the conifer trees but plant them parralell to them, hence cutting back to the trunk on one side.
But I`m not sure what distance to leave between the row of conifers and the newly planted laurel hedge.

Not a lot grows under or near a conifer hedge, and you will find it difficult to water/feed the laurel without the conifer taking the goodness away first. Taking the top out of the hedge will not help planting in front of it - even two to three metres away and I can only recommend you ditch the conifer completely, including stump grinding, before planting your laurel, especially if you wish for a healthy and bushy laurel hedge.

I agree, you will probably be wasting your money on new laurels if you plant in ground where conifer roots have been sucking away the goodness for years. Even if the conifers were completely removed you would need to replenish the soil before replanting. If the hedge is still growing your laurel will not thrive. Dianebulley's roses are well away from the old trees, which have actually been removed anyway. If yours are still in place the situation is different.