Owning your own home

2-up, 2-down in Brighton. Ex council. 1960s build. Freehold. Want to weigh up options of staying on here for a few years or moving elsewhere given impending family enlargement so we're giving some thought to extending.

Is there a general rule of thumb on easiness/best value as an investment to extend outwards or upwards?

Extend into the loft: large enough for a massive master bedroom (the size of the whole floor space nearly) with en suite and a small landing. Currently the ceiling doesn't support any weight so would obviously need some substantial work. It then becomes a three storey house with 3 beds. Master bathroom still quite small. 2nd and 3rd bedrooms decent size. Retains fairly substantial rear garden. Other houses in the street have done the same.

Extend into the garden: allows a good size 3rd bedroom and we would likely extend the bathroom to a good size to boot. Doubt we would have space to ensuite any of them. Though we would do both floors (again, houses in the same street have done the same) so would have either an extra reception room or a very large kitchen/diner. If the latter we could maybe plumb in a downstairs toilet or have an extra storage area or workshop or something.

To make matters more fiddly the garden, although long, is terraced so if we extended outwards we would likely end up rght up against the wall of the first terrace so to have any daylight in the kitchen/new reception room we would need to do some structural work to the garden as well.

This is all theory at the moment - haven't applied for any planning permission. We don't have any particular budget in mind but could get our hands on whatever was needed. If we extended we would likely live here for a good many years to come.

Is there any received wisdom on which type of extension would make for better value to the investment?