Will taking an extra year to complete B.Tech. affect admission to M.Tech.?

Once you have completed your B.Tech, you are eligible for attending the GATE examination. Even if you have completed the same in 5 years, there will not be any problem to attend GATE.
Your IIT admission will depend on the score you have obtained in GATE. If your score is suitable to acquire a seat in IITs you will get it.

Let me provide you straight answer to your query regarding your late completion of B. Tech taking five year time from your end in lieu of its normal duration of four years.
In no way, it will hamper you to write your GATE examination. Now depending upon your GATE percentile score followed by your interview from the admission committee will determine your success rate for the branch of ECE.

For some institutions this can be a problem. They might not accept you based on this. But majority of institutes will take you if you score good marks in Gate examination. There is nothing wrong in failing, but we must learn from those failures.

As mentioned above, you have completed your B.tech in 5 years so, you are eligible for the GATE examination. Your M.tech admission to IIT depends on your GATE percentile score. So, there is no need to worry about it. There are some eligibility criteria for admission to M.tech IIT which are as follows:-
1. BE or B.tech in any engineering discipline with 60% aggregate.
2. GATE score is mandatory.
Therefore, You should focus on the GATE examination. If you get a good score then definitely you will get a seat.

In most of the engineering graduation programs there is a provision of completing it in a maximum of 6 years in place of 4 years for those students who have some papers remaining to be completed. So this is a provision that many students avail and is a common thing.
So in your case also as such there is no problem of getting admission anywhere based on your GATE or B.Tech. score.
Only thing is if there are candidates having same scores applying for admission then those who did their B.Tech. in stipulated time of 4 years would get preference. Other than that there is no problem.

Went through the information brochure for M.Tech admissions at IIT Roorkee. There it mentions that those with any back papers or supplementary need to complete them before the date of registration. Other details included having 60% marks or a 6.00 CGPA out of a scale of 10 and a valid GATE score. Thus it is clearly apparent that those with back papers, supplementary or a year extra are eligible to apply for an M.Tech program in IIT Roorkee as long as the other conditions are fulfilled. The same should be true for other IITs also. At IIT Roorkee, those with a CGPA of 8 or more out of a scale of 10 can directly apply even without the GATE score.