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I'm in the process of doing this. One thing I've discovered is that planning permission is more restrictive if you're a top floor flat than if you are a full house so it is worth looking into what you will be able to get away with in terms of size.
I found very few flats post-conversion to compare it to so it's difficult to see what the increase in value would be. I suspect it won't be hugely more than the cost of the conversion and I'm not convinced it would be worth it if you're just looking at financial return.
I went ahead with it for a few reasons. The roof needs some serious work so I'd be having scaffolding up and all the disruption anyway, this is what started it off. The increase in space will probably also mean I can get another 5 years plus of living in the flat comfortably (girlfriend will be moving in soon) and it will also give me the option down the line of moving the kitchen which is currently very small.
The top flat three doors down from us is having a loft conversion. This has set my mind ticking for future possibilities.
We are leasehold so that's an obstacle, but not impossible.
My thoughts are more about what the potential market is for a 3 bed maisonette without a garden, and how much would that add to the value of the flat.
If the project cost £60k (wild guess), then the uplift in value would have to be significant to make it worth the hassle.