5 Marla = 25x50 = 1250 sqft. With some open spaces for porch, lawn etc., constructed area may be around 1000 sqft.
2600/sqft is too high a rate. If you purchase materials yourself and supervise the construction, you may get house with decent finishing in around 1300-1500 per sqft. Means construction cost for single story 5 Marla should be around 1.3-1.5 million
Rest, if you spend too much on tiles, marbles, washroom accessories, ceiling etc.then costs can go up high considerably.