Extending a bed from 1m to ...more

Down one side of the garden is a strip of border, 1m deep next to a fence. It is currently filled with scrappy bits of low-medium height perennials between a mahonia and a Camellia.
About 2 - 2.5 metres away from the fence, in the lawn is a crabapple tree growing out of a very poor lawn - due to shade and being bombarded with slimy crabapples in the autumn!
Whilst I've been digging in this area, I've noticed that it has a hard pan in places about a spade'ish deep. I know there's a soakaway in this area so I'm guessing that could also explain the dip in the lawn in this area.
I want to deepen the bed away from the fence to about 2 - 2.5m, I thought I'd bring it wavily round the crabapple and plant some fill in shade loving shrubs at the back, along with some clematis that I've already put in and move the smaller stuff to the front and middle where they'll catch some sunshine.
I had a brainwave today, instead of cutting the shape, removing the grass and digging it over, why don't I buy in a load of topsoil and just dump it on top since this area is lower than the surrounding garden and has this hard pan issue in places?
Anyone ever tried this cheat and did it work out for you?
Thanks,
TT
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I think the hard pan would still cause an issue Tetley - unless you're putting a helluva lot of topsoil down. If it was me - I'd want to address that first before planting anything, especially things that need decent drainage, like your clematis
Loads of grit and shingle dug into the worst parts, and some well rotted manure would really help before putting the topsoil in. Perhaps others may have good experience of dealing with something like that though.
I live in west central Scotland - not where that photo is...
My back garden was just grass when the children were young but once they outgrew it I bought a huge load of topsoil and created scalloped beds round the perimeter by simply dumping it on top of the grass. Worked a treat though I didn't have hard pan issues.
I sort of did this, this time last year and it worked fine. I worked out the shape of the border I wanted, lifted the turf and dug over the soil. Then I buried the turf and shoved a load of mulch over the top.
it was fine to plant in this spring. 95% of the grass had died and rotted away. The only bits I had to remove by hand were bits the cats had unearthed.
Thanks for the feedback, you all confirmed what I was thinking - if I don't address the hardpan before I do it, it won't be addressed!! If I chuck topsoil on the top it'll be ok and if I actually do some prep it will probably be better.
Thanks guys - that's a Spring job!
"Dear hardpan....."
Chuckle, Steve
Steve
I live in west central Scotland - not where that photo is...
Thing is, I haven't really worked out what the hardpan is yet - could be roots, could be foundations for the fence, could be a soakaway. If it were only clay I could dig through but if it's a soakaway, I've no choice. It is within root range of a 70'ish foot American Walnut tree next door and there are also roots from an old self seeded sycamore that tried to come back to life this year but I've SBK'd it to death.
I need to do some test holes when it's not quite so er... soggy!
Dear Hardpan, I'm coming to get you ...