Turning landing into 4th bedroom to sell

[ToddlerTerror]

I always thought I would live in my house until my dying day but over the Christmas holidays, the PERFECT property came on the market for us. It would be a huge stretch financially so trying to work out how to add extra value to ours.
House is a 3 bed detached cottage with 2.5 acres land and stables and we have completely renovated it over the last 4 years.
I was reading the old particulars from when we bought it and it said about the possibility of adding a forth bedroom on the landing. The landing at the top of the stairs is rather big and a bit of a wasted space. I was wondering if it would be worth putting up a stud wall to create an office/single bedroom. It would measure about 9ft by 6ft. We could do it in a weekend but it would mean the hallway would be dark as no natural light if the doors are all closed but that wouldn't be often.
Would this dramatically add value to the house/ make it more appealing? Or do you think it would make it feel a bit cramped?

It's the norm in my area to have 4 beds and one bathroom, OP has a shower room and bathroom. Obviously an extra bathroom is preferable, as is a large landing filled with light. Unfortunately, some people need the extra room and have to forego this luxury.

Someone needing a 4 bed that buys OP's house, if converted, is unlikely to think "Oh I'll make 2 kids share and open up this amazing landing".

I 100% get that someone only wanting a 3 bed would prefer the bigger landing though, but realistically it would probably sell for less.

I'd leave it as it is and let the agent suggest to buyers that it's an adaptable space. Let them find out how to adapt it. Many buyers may not want to, a dark landing and a poky room would put me off for one.

I'm surprised the EA said this too @ToddlerTerror and I'd definitely want to get more than one opinion and also to establish from them what difference in selling/sold price there would be between a three and four bed property.

Apologies if I've missed it @ToddlerTerror but I think in addition it would be helpful to see the ground floor plan to establish whether the living space of the cottage would support a fourth bedroom....

If you did decide to do it, look into solar tubes. They work with most types of roof, and allow you to bring light into dark hallways.

I would still opt for a window, though, a small window is not hard even in a very thick wall (have done this several times).

We had a 3 bed cottage. It was pretty large, well over 2000 square feet, and over 2 acres of garden and woodland, but because it only had 3 bedrooms, the price ceiling was very low and it took many years to sell as it was too big for a couple but too few bedrooms for a family (that wouldn't apply in London, but in rural areas families typical look for 4plus bed houses). Do you have a family friendly downside space? A 4 bed house with large grounds would probably need a sizeable kitchen diner and seperate good sized reception room.

@ToddlerTerror The agent has said it isn't worth doing because s/he wants the property on the books asap so they can sell it and get the commission in the bank! They don't want the delay whilst you do the works, and it doesn't make that much difference to them if they get say 1% of 500k of £600k (just plucking numbers out of the air) whereas it could make a big difference to you.

This.

Perhaps consider saying to another that you were thinking of marketing in 6 months, due to whatever issue, and in the meantime are going to do the work. Get the valuation based on the proposed 4 bed, then compare to whatever the other valued at for the 3.