Fixed somthing my landlord wanted?

Use concrete contractors to fix.

No- repair & deduct is only for repairs needed to essential services.

Unless there was a written agreement to do so then he has no legal obligation to pay you. 

I'd keep this in mind should he ever ask you again for assistance.

Should have got it in writing.

This structure is for you though right?

It probably does enhance the value of the landlord's property overall, yet the person who is getting full use out of it is yourself and family. By the time you move out, whatever was built or repaired will be second hand, more or less.

You didn't mention who suggested this be built, but I'm wondering if it was your idea and the landlord just went along with it?

However that being said, he certainly shouldn't have stated that he will compensate you for it and then do a turn around. That's not very fair or professional.

Legally the landlord has no real obligation to reimburse you for any of costs incurred at the moment. However if you were willing to take the matter to small claims tribunal, the magistrate may consider words expressed, to form a legally binding contract.

It doesn't cost much to file a date for court appearance and neither side gets to have lawyers, so might be worthwhile. Yet the landlord won't like you for this move, so it could create future drama or difficulty.

Edit:

As per your update, you can't unilaterally mark it as a repair and self deduct from the rent. The landlord could have grounds to evict you. That's for a small claims magistrate or tenancies tribunal to determine.

Mate that's how landlords are.