When you buy a Car, you will ask the seller to demonstrate the working to your satisfaction before taking delivery.
If you do not know to drive you may take a friend to test drive.
Say even if you drive, you will take a mechanic or an expert to assess the condition of engine, transmission, suspension, battery etc., of which he has specialised knowledge.
No seller of car refuses an inspection by a mechanic. Then why not the same case with a flat, if he has nothing hide.
Most Developers /Builders welcome inspection by third party thus enhancing their reputation of transparency and showcasing their good quality.
As a consumer you have all the rights to inspect yourself or through a friend or an expert of your choice before taking over your property.
Any refusal to allow inspection, should be viewed with suspicion leading to a stronger case for third party inspection.
A consumer can assert his rights to confirm whether he is getting what was promised to him before taking to legal recourse as final resort.