Are you referring to the UK laws here?

If the basic amenities are denied then you always can fight for it through the authorities concerned. 

You can make a representation with the local civic body and try to get relief by invoking the properprovisionsoflaw. . 

As per government construction laws each floor should have toilet, no matter how old the construction is.

If they are not allowing you. On your child disability basis you get a permission to built new toilet if empty Space is available on your floor.

Kindly obtain copy of building sanctioned plans from muncipal corporation it must be containing provisions that .every Flat should have one toilet 

2) The area of a bathroom shall not be less than 1.8 m2 with a minimum width of 1.2 m. The floor area of water- closet shall be 1.1 m2 with a minimum width of 0.9 m. If bath and water-closet are combined, its floor area shall not be less than 2.8 m2 with a minimum width of 1.2 m.

3) 

• One of the walls should have an opening to the open air. Minimum ventilation or window space should be provided measuring up to 0.37 sq metres.
• A bathroom must always be over another bathroom or washing place or the terrace space and not over another room. Watertight floors can be an exception to this rule.
• The seat should be made of non-absorbent material.
• Bathrooms should be enclosed by partitions/walls, provided with an impervious surface with a height prescribed as not less than one metre.
• The floor covering should be impervious too but sloping towards the drain and not towards any other room or balcony space.
• A room provided with a water closet is to be used as a toilet only. These rooms must be provided with flush cisterns.
• If there is a toilet on the terrace with a height of 2.2 metres, it should be counted in the Floor Area Ratio (FAR).
• In the absence of a sewage outlet, a septic tank must be provided.

This is weird. How can they tell you that they cannot fix it till March when you have problems. You should write to the civic authorities and inform them of the problems you are facing. If they do not respond then file a petition against them.

No there are no such laws regarding min. No. Of toilets.

Healthy, convenient and hygienic life is granted to every citizen under Article  21 of Constitution of India. To avoid delay of law, file a direct writ in the High Court of State where the property is situated. You will get favourable direction in a week. There is not guarantee that they will do it by March, better to get direction from High Court .

Dear client,  One toilet should be provided for every 1-6 guests. Two toilets should be provided for every 7-12 guests. Three toilets should be provided for every 13-18 guests. Each of these toilets should also have a washbasin provided nearby.

You can file case under consumer laws for deficiency of service and the toilet repaired urgently