By lean to i take it you mean front, side and rear using the house gable as the other side?

Double brick walls, lean to roof, up and over door etc?

If you said 6m long X 3m wide you would be looking at around

Compacted hardcore base 8inch deep £200
concrete slab 6inch thick £300
3000 bricks approx £1000
3 tonnes sand approx £90 if bagged
25 bags cement £70
Roof timbers approx £300
Roof tiles, lats, felt approx £500
Up and over door £500
Electrics and misc £500

all plus labour.